`/data` and `/sdcard` use the same block device?

Question

My OnePlus 3 is in a pretty weird situation. I deleted the contents of /sdcard on an adb shell while my phone was running TWRP via:

# rm -r /sdcard*

I also deleted the hidden files files but they are in a different tar ball than the non-hidden ones and are already restored. For this, I pushed their tar file to my phone and extracted it in an adb shell. However, pushing the substantially bigger tar ball of non-hidden files over to my phone failed.

Upon investigation, I found that apparently /dev/block/dm-0 is mounted both in /data and in /sdcard:

~ # df -h
Filesystem                Size      Used Available Use% Mounted on
tmpfs                     2.8G     24.0K      2.8G   0% /dev
tmpfs                     2.8G     24.0K      2.8G   0% /tmp
/dev/block/sda3         248.0M    272.0K    242.6M   0% /cache
/dev/block/dm-0          52.7G     36.9G     15.7G  70% /data
/dev/block/dm-0          52.7G     36.9G     15.7G  70% /sdcard

The contents of /sdcard don't make up for 36.9 GiB. It's only TWRP (a few GB) and the hidden folders (about 1 MB).

/sdcard # ls -la
__bionic_open_tzdata: couldn't find any tzdata when looking for localtime!
__bionic_open_tzdata: couldn't find any tzdata when looking for GMT!
__bionic_open_tzdata: couldn't find any tzdata when looking for posixrules!
drwxrwx---    5 media_rw media_rw      4096 Aug  1  1970 .
drwxrwxrwt   23 root     root           720 Aug  1  1970 ..
drwxrwxr-x    2 media_rw media_rw      4096 Jan  6 00:22 .AzPlugin
-rw-rw-r--    1 media_rw media_rw         5 Sep  1 14:22 .appodeal
drwxrwxr-x    6 media_rw media_rw      4096 Jan  6 00:22 .estrongs
-rw-rw-r--    1 media_rw media_rw        72 Oct 19 16:06 .userReturn
drwxrwxrwx    2 root     root          4096 Aug  1  1970 TWRP

The weird thing is that /data – even though the same device is mounted to it – contains a lot of files and folders /sdcard doesn't:

/data # ls -la
__bionic_open_tzdata: couldn't find any tzdata when looking for localtime!
__bionic_open_tzdata: couldn't find any tzdata when looking for GMT!
__bionic_open_tzdata: couldn't find any tzdata when looking for posixrules!
drwxrwx--x   38 system   system        4096 Jan  5 22:18 .
drwxrwxrwt   23 root     root           720 Aug  1  1970 ..
-rw-------    1 root     root             2 Aug  1  1970 .layout_version
drwx------    2 root     root          4096 Aug  1  1970 adb
drwxrwx--x    2 system   system        4096 Aug  1  1970 anr
drwxrwx--x  107 system   system        4096 Jan  5 23:10 app
drwx------    2 root     root          4096 Aug  1  1970 app-asec
drwxrwx--x    2 system   system        4096 Aug  1  1970 app-lib
drwxrwx--x    2 system   system        4096 Aug  1  1970 app-private
drwxrwx---    3 media    audio         4096 Jan  5 22:13 audio
drwx------    5 system   system        4096 Jan  5 23:10 backup
drwxr-xr-x    2 shell    shell         4096 Aug  1  1970 bootchart
lrwxrwxrwx    1 root     root            45 Jan  5 22:13 bugreports -> /data/data/com.android.shell/files/bugreports
drwxrwx--x    2 system   system        4096 Jan  5 22:13 connectivity
drwxrwx--x    5 root     root          4096 Jan  5 22:15 dalvik-cache
drwxrwx--x  245 system   system       12288 Jan  5 23:10 data
drwxrwx--x    3 system   system        4096 Jan  5 22:13 dpm
drwxrwx---    3 drm      drm           4096 Jan  5 22:13 drm
drwxrwx--x    2 system   system        4096 Aug  1  1970 fota
drwxrwx---    2 system   system        4096 Jan  5 22:18 fpc
drwxrwx---    4 system   system        4096 Jan  5 22:13 fpc_images
drwxrwx---    2 system   wifi          4096 Aug  1  1970 hostapd
drwxr-x--x    3 root     root          4096 Jan  5 22:13 local
drwxrwx---    2 root     root          4096 Jan  5 16:08 lost+found
drwxrwx---   35 media_rw media_rw      4096 Aug  1  1970 media
drwxrwx---    3 mediadrm mediadrm      4096 Jan  5 22:13 mediadrm
drwxrwx--t   39 system   misc          4096 Jan  5 22:13 misc
drwxr-xr-x    3 nfc      nfc           4096 Jan  5 22:18 nfc
drwxrwx--x    2 radio    radio         4096 Jan  5 22:13 oemnvitems
drwx------    2 root     root          4096 Jan  6 00:19 property
drwxrwx--x    2 system   system        4096 Aug  1  1970 resource-cache
drwx--x--x    2 system   system        4096 Aug  1  1970 security
drwxr-xr-x    2 system   system        4096 Aug  1  1970 shared
drwx------    2 system   system        4096 Aug  1  1970 ss
-rw-------    1 root     root      11236116 Aug  1  1970 stock_boot_fbcb35a6c03d1c2da5dbb1784d5642fcfda6b4cd.img.gz
-rw-r--r--    1 root     root     100663296 Jan  5 22:15 su.img
drwxrwxr-x   18 system   system        4096 Jan  6 00:19 system
drwx------    2 system   system        4096 Jan  5 22:18 theme
drwx------    2 system   system        4096 Jan  5 22:13 time
drwxrwx--x    6 system   system        4096 Jan  5 22:13 tombstones
drwx--x--x    2 system   system        4096 Jan  5 22:13 user
drwx------   12 system   system        4096 Jan  5 22:13 usf

Why is this?

When I looked for what took the storage space, I found that there are files which used to be on the (emulated) SD card but which I deleted (which is why /sdcard is so empty) before trying to push the tar ball over to my phone:

/data # du -h . | grep '[0-9\.]\+G'
4.3G    ./media/TWRP/BACKUPS/8b32aa5/2016-09-25--16-41-47
4.3G    ./media/TWRP/BACKUPS/8b32aa5
4.3G    ./media/TWRP/BACKUPS
4.3G    ./media/TWRP
4.0K    ./media/Android/data/com.google.android.GoogleCamera/files
8.0K    ./media/Android/data/com.google.android.GoogleCamera
2.7G    ./media/Download
1.3G    ./media/MEGA/MEGA Offline/Music
1.1G    ./media/MEGA/MEGA Offline/Documents/Uni/Modules
1.2G    ./media/MEGA/MEGA Offline/Documents/Uni
1.2G    ./media/MEGA/MEGA Offline/Documents
2.5G    ./media/MEGA/MEGA Offline
2.5G    ./media/MEGA
15.8M   ./media/Pictures/9GAG
1.4G    ./media/Pictures
1.8G    ./media/TitaniumBackup
32.0G   ./media
4.0K    ./data/com.qualcomm.qti.GBAHttpAuthentication.auth/cache
4.0K    ./data/com.qualcomm.qti.GBAHttpAuthentication.auth/code_cache
12.0K   ./data/com.qualcomm.qti.GBAHttpAuthentication.auth
4.0K    ./data/com.oneplus.GpioSwitch
4.0K    ./data/com.google.android.GoogleCamera
12.6M   ./app/com.google.android.GoogleCamera-1/lib/arm
12.6M   ./app/com.google.android.GoogleCamera-1/lib
12.2M   ./app/com.google.android.GoogleCamera-1/oat/arm
12.2M   ./app/com.google.android.GoogleCamera-1/oat
47.4M   ./app/com.google.android.GoogleCamera-1
3.8G    ./app
36.9G   .

What can I safely delete? Can I wipe "Data" in TWRP without apps malfunctioning? What happens when I delete the contents of /data/media? Are those hard links of the files I deleted on /sdcard?

I pushed that tar ball several times over to my phone, today, extracted it, deleted it, messed up, deleted the contents of /sdcard, and started over. However, this time, which thus far appeared to be the last time, such weird behavior occurred.

Edit: This question was closed as a duplicate but linked question is entirely different. Just like... Read my question before you close it, please? The linked question is about someone who doesn't know what a symlink is. Mine is about how it can be that data in two locations that have the same device mounted to them can have different contents. The question was closed by someone who doesn't know about symlinks nor about mounting.