Internal memory explanation I have a Moto E 2º Gen, 8gb internal memory.

If we substract the Android OS requirements, i'm left with 4.98 Gb, which appears as available internal memory.

It shows 4.11Gb used memory, however, below the phone shows the following:

Apps: 2.21 GB

Images: 8 KB

Videos: 0 B

Audio: 120 KB

Other: 99.98 MB

Cache: 1.99 MB

which adds up  close to 3.2GB, so: 3.2GB != 4.11GB. What's going on with those missing 900MB?

Another hints that troubles me comes from plugging the phone to a PC via USB:

Internal memory shows only 342MB available, and the rest all occupied, but when i access the internal files, all files togheter only weigh 224MB!!, and there are no hidden folders.

Can someone explain why this happens, and what are the REAL numbers to take into account.

How could it be that there are hidden files that i can't see even with Windows option "see hidden files" activated?

This is a specific question, but i've seen this in other android devices, so maybe the answer could be in the OS and the way it manages data, more than a device specific issue. Maybe some internal OS memory is not being shown by the device?
I'll try to be straightforward. My answer won't give you a formula explaining why this exact amount of memory is missing from your phone, but it'll hopefully give you an insight about where it's being used and ... why it's needed.

Think of a storage as a copybook, in each page you can store certain amount of data, the technical term for that is a block and its size varies from a system (hardware + software) to another.

Now imagine that each of your files is stored in one page of that copy book, let's say you've got 24 pages and 14 files. You might think that the remaining pages are 10 pages (24 - 14 = 10) but that's not right.

You see, in order for your system to know the location of each file it has to store somewhere the location of each single file, it's like an index containing the number of the page for each file. This index itself might take some pages from your copybook and so does the index used by a real file storage system (FAT, FAT32, NTFS...)

This is probably where your missing storage is being used. Just check what file system is implemented on your phone, make some research and you'll get some more in depth explanations (may be some formula along the way).

Cheers!