Android's internal storage is made up of many symbolic links. They point towards different locations of your phone's storage.
/mnt/sdcard internally links to
/data/media. This directory just contains your user data but no applications or application data.
/data/ contains multiple directories containing the apps (
/data/app) or the app data (
/data/data) that all contribute to the used storage.
What you see using
df is the total storage statistics of the whole partition, even though you run it on
Citing the Linux manual: df - report file system disk space usage.
This means it takes into account all files and folders on that partition even though you are using a directory of the partition as the parameter.
df returns the statistics for the complete partition, but you are just looking at one directory inside of it.