git pull and GIT_DIR

It appeared that

a) cd /path/to && git pull git/https://....

will fetch and checkout latest changes to working tree. This is as expected. On the contrary

b) GIT_DIR=/path/to/.git git pull git/https://....

will only update git repo, but leaving working tree untouched. Subsequent git status and git diff will indicate unstaged changes, apparently a commit that will revert the updated git repo. I have to do 'git reset --hard' to really update the working three.

I'm not sure if this is an intended behavior or I missed something.