I just noticed that, when I offer pip a directory containing a development version of matplotlib, it will install it, even though I do not use the --pre flag. For example this directory:

https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com/

contains uploaded development versions of matplotlib, e.g.

https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com/matplotlib-2.0.0+3158.g7b79174-cp27-cp27mu-manylinux1_x86_64.whl#sha256=643db393739b13ac93f6519a1dff8af41585dd5462a363f6aa1a923a68b783c1

If I do:

pip install --find-links=https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com matplotlib

I get:

Collecting matplotlib
  Downloading https://7933911d6844c6c53a7d-47bd50c35cd79bd838daf386af554a83.ssl.cf2.rackcdn.com/matplotlib-2.0.0+3158.g7b79174-cp27-cp27mu-manylinux1_x86_64.whl (14.3MB)

I think this is because pip is interpreting the +3158.g7b79174 as irrelevant version decoration, and therefore thinks the real version is 2.0.0, and that this is a stable version.