One reason can be remembed locations.
You can clear the cached locations by issuing following command:
hash -r
SIDENOTE: Instead of which, using type command, you can see the hashed location:
$ type pip
pip is /usr/local/bin/pip
$ pip -V
pip 1.5.6 from /usr/local/lib/python2.7/dist-packages (python 2.7)
$ type pip
pip is hashed (/usr/local/bin/pip)