I propose to add at least a statement like e.g. "In conformance to IEEE Std 1003.1™, 2013 Edition; 4.12 Pathname Resolution".

Because I had the same thought of a bug at first view, this because I did not find any hint in e.g. docs for 2.7.11. 

The reason to handle this thoroughly in my projects is the application of a path-matching library for generic unit tests, e.g. for bash scripts which require intensive PATH resolution. This has to be applied by the users of the library.

See "https://pypi.python.org/pypi/pyfilesysobjects", and  "https://pypi.python.org/pypi/epyunit" which requires intensive pattern matching of application provided pathnames, e.g. when it comes to automatic split of actual used PYTHONPATH items for a specific function/method, module, or package.

E.g. the user provides an 'intentional casual' pathname for drop-in unit tests(see epyunit),
  "os.sep + context.sys.path[x] + os.sep + 'rel-module-path'",
due to mixed relative and absolute paths resulting in leading 
  "os.sep + os.sep".  
The pattern match than fails, but it is not immediately clear for which reason.