pip 8.1.1 is the version of python-pip from the default Ubuntu 16.04 repositories. When installing a package with pip set the HOME environment variable to the home directory specified by the target user's password database entry by using the option -H .

In the question you linked to two different versions of pip were installed. To find out why pip -V returns a more up-to-date version of pip (pip 9.0.1) which is also installed, run the following commands:

sudo -H pip -V

This command returns pip 8.1.1 from /usr/lib/python2.7/dist-packages (python 2.7) in a default Ubuntu 16.04 installation. The command sudo -H pip3 -V returns pip 8.1.1 from /usr/lib/python3/dist-packages (python 3.5) in a default Ubuntu 16.04 installation.

which python3

This command returns /usr/bin/python3 in a default Ubuntu 16.04 installation.

which pip