[asianexpress]

Not sure if this is the primary reason, but:

"Look for specified patterns in the tracked files in the work tree"

http://git-scm.com/docs/git-grep

Presumably, this would exclude a lot of assets or libraries that aren't part of your branch/repo, don't know what the OP's setup was (are those 11K+ files all tracked?).

[raylu]

If you have a compiled project, files that aren't tracked include all of your intermediate .o files, for example. If you `grep -r` the wrong way, it may traverse the .git directory.