Here's the detailed problem: http://stackoverflow.com/questions/11560056/pip-freeze-does-not-show-repository-paths-for-requirements-file
pip's ability to discern where the install came from is limited after the fact using pip freeze.
pip internally relies upon setuptools to find all the installed distributions.
pip then checks to see if any of them are "editable"/-e vcs installations.
if editable, then it will print out the vcs url.
if not editable, then just the name=version spec.
btw, since you asked about "-e" in the stack link, "-e" or "editable" is an enhancement of what setuptools called a "develop" install. this is an "installation" that does not occur via copying packages into site-packages.
but rather a link of sorts is made in site-packages back to the source location of the project.
that way you can "develop" or "edit" the project live while it's installed.
for "-e" vcs installs, pip uses a "src" subdirectory of the virtualenv or sys.prefix to clone the project.
if you install from a vcs url w/o "-e", pip builds a distribution locally and installs from that, and at that point pip freeze (i.e. setuptools) doesn't know it came from vcs.
Fixed issue #609 - method to detect VCS backend