My HTC One M8 internal memory is almost full (15.40 out of 16). The phone says it's mostly applications. But when I connect it to my computer, go to Internal Storage and add the sizes of all directories, the total is about 3 gigabytes.
Devices have different partitions for different purposes. Data storage is bigger because you will need to store your pictures etc. BUT the partition that your device uses for apps is always limited. You might want to move some apps to sd if you are able to. For example here is the partition table to a Samsung Galaxy S5:
aboot -> /dev/block/mmcblk0p6 apnhlos -> /dev/block/mmcblk0p1 backup -> /dev/block/mmcblk0p18 boot -> /dev/block/mmcblk0p15 cache -> /dev/block/mmcblk0p24 carrier -> /dev/block/mmcblk0p25 dbi -> /dev/block/mmcblk0p4 ddr -> /dev/block/mmcblk0p5 efs -> /dev/block/mmcblk0p12 fota -> /dev/block/mmcblk0p17 fsc -> /dev/block/mmcblk0p19 fsg -> /dev/block/mmcblk0p9 modem -> /dev/block/mmcblk0p2 modemst1 -> /dev/block/mmcblk0p13 modemst2 -> /dev/block/mmcblk0p14 pad -> /dev/block/mmcblk0p10 param -> /dev/block/mmcblk0p11 persdata -> /dev/block/mmcblk0p22 persist -> /dev/block/mmcblk0p21 recovery -> /dev/block/mmcblk0p16 rpm -> /dev/block/mmcblk0p7 sbl1 -> /dev/block/mmcblk0p3 ssd -> /dev/block/mmcblk0p20 system -> /dev/block/mmcblk0p23 tz -> /dev/block/mmcblk0p8 userdata -> /dev/block/mmcblk0p26
Hope this helps.