In the first case, how does the package maintainer know which version of libc to use? It should use the one that my system uses (because I might also use other libraries that are provided by my system).
The libc version(s) to use when creating python packages is standardised and documented in a PEP, including how to name the resulting package to describe the libc version. Your local python version knows which libc version it was compiled against and reports that when trying to install a binary package. If no compatible version is found, it tries to build from source. If you are doing something 'weird' that breaks this, you can always use the --no-binary flag to force a local build from source.
