Storage memory low

[Guest jazz198X]

It says so every now and then when I start up the camera. What bothers me the most is that of these 87M of files, I can only account for about 51M worth through file explorer (total commander) if I look at the properties of ALL files and folders except the ones in ROM. Tools like MemMaid, ClearTemp, AutoClean etc clean up about 0.5M of waste, so where is the rest of the 30M that should be free??? I have practically all apps installed to my storage. My documents has 1M, Program files 16M, Application data 4M etc ... I see this problem everywhere on the internet, with no working solutions or more importantly, explanation of why this is happening and what (not) to do to to avoid it.

This does not occur with installing more applications, as after I did that I still had about 20-30M free. This occured after using the phone for a while (say a couple of months), as if it was about some temp files, but they should still be visible through file manager right?