Surprising semantics of "git add file"

Hi!

Reading the manual, I realized that "it is the way it is (it works as documented)", but to me it's surprising, meaning: I feel it's wrong: (git-2.16.4) I have staged several files, and then I do "git add -interactive file" to add some selected changes from file. After that a "git diff --cached file" just shows the changes added interactively, but when I "git add file" to commit those changes), even the unstaged changes from file are committed. I feel this is inconsistent: At least "git diff --cached file" should behave like "git commit file", meaning "git commit file" should be fixed IMHO. Or are there any reasonable use cased for that?

Regards, Ulrich Windl