One way to circumvent this btw would be to also add/use the 

--python

 flag for 

virtualenv

Notice that when creating the venv , the cmd that is used is basically pythonx.y -m virtualenv ...
By definition this will create a new venv based on the python that executes the venv.
With all that said, it might be there is a bug in virtualenv and in some cases, it does Not adhere to this restriction