When confronted with this mysteries you should always search the absolute path of what you're executing. You obviously are executing two different commands, and in order for that to happen the PATH environment variable of your user is different than the one of your root user. You can compare them by executing:
$ echo $PATH
$ sudo su
# echo $PATH
You can search through those directories and find the different pip scripts. My bet is that you have a local pip installation that lives in one of the directories mentioned in root's $PATH.
If you want to know the version of the pip installed by your package management tool, just issue the following command:
$ dpkg -l | grep python-pip
Run the following command if you want to know what files where installed by this package.
$ dpkg -L python-pip
Having found the culprit using this methods, you can now uninstall the version you don't need.