These are the standard packages, and will always follow with that version of Python and Virtualenv.
distribute is pretty self-explainatory. It's necessary for pip. Distribute also contains setuptools, but inside the package so not recognized with pip freeze. For more information about what it actually does check out your env/lib/python2.7/site-packages/distribute-0.6.31-py2.7.egg.
wsgiref is actually a part of the standard library, but isn't recognized as so. There's a bug report on it, and it's fixed in Python 3.3+. Read more about it in Why does pip freeze report some packages in a fresh virtualenv created with --no-site-packages?
I can't find out why argparse is there though, but my guess is because it's a dependency or something like wsgiref. Finding package dependencies in Python can be a bit hacky/painful though, especially if it's already installed in your virtualenv.
