Reset sometimes updates mtime

I'm seeing some behaviour with git reset that I find odd. Basically if I do

git fetch && \ git reset --hard simple-tag-that-points-to-the-current-commit

sometimes the reset will update the mtime of all files and directories in the repo and sometimes it will leave them alone. Changing it to

git fetch && \ git status && \ git reset --hard simple-tag-that-points-to-the-current-commit

Cause the mtime update to reliably not happen.

Bad thing is that I am relying on the mtime updates not to happen if the files don't actually change. Is this an assumption I can safely make? If it is, then I'll debug further (e.g. I don't even know yet if the file gets rewritten or just touched, why the index gets updated as well etc.).