if you run tox -v you should see which virtualenv is picked up for creating the env. Also, which tox versions are you using? On which operating system? The issue you report is unknown to me, haven't seen it popping up so without more context there is not much we can do to help.
I've occasionally installed tox using pip2 instead of pip3.
And my python2.7 environment is indeed pretty ancient.
With python3 everything works fine.