Python - Using the django that is installed in the virtualenv
virtualenv is a tool to create isolated Python environments.

It doesn't have anything to do with your code and static_url settings, only thing different is what packages you have there and what django core version you are using.
I've installed Django 1.8 in the virtualenv and I'm trying to use the static files from it.
For Example, if I want to edit the header color of the admin base.html, It keeps using the django global file (1.7) even though I'm working with my virtualenv on.
Doesn't my STATIC_URL = '/static/' use the django that is currently running in my virtualenv?
Sorry for the bad English