I'm not sure why, but using TortoiseGit->Diff from a Windows Explorer window shows every file as being modified even when none in the directory has changed. A command line call to 'git diff' in that same directory shows no modified files, but running TortoiseGit->Diff brings up a dialog box with a list of all the files in that directory and sub-directories and indicates that all these files have been modified.
Here is an example, of the statistics that one of these diff dialog boxes shows: 'line: 0(+) 0(-) files: normal=0, non-versioned=0, modified=42, added=0, deleted=0, conflicted=0' As you can see, TortoiseGit->Diff knows that nothing in the files has changes but it is convinced they've all been modified.
This behavior is consistent across any git repo that I'm working on or even just any repo I clone from GitHub.
Here is the version I'm using on Windows 7
TortoiseGit 18.104.22.168 (C:\Program Files\TortoiseGit\bin\)
git version 2.1.0 (C:\cygwin64\bin)