Diablo 1 HD Hellfire Mod | Awake

I opened my warrior's save files (and backup) in the regular editor, and compared them. They differed slightly less than full. How engine can understand all this stuff?

What do you call "the regular editor"?
Anyway, the engine "understands all this stuff" because it has been programmed to do so...

Notepad++

I mean, when two files are very different, even when you move the HP bottle in the another slot.

Perhaps they are compressed or encrypted.
Anyway it seems obvious they are not intended to be easily editable.