> what is the difference

sys.platform is defined by code path taken while compiling. it is hardcoded and really represents the os abi used of a *supported* platform, it is to be trusted.

while sysconfig.get_platform() try to guess from various sources with more or less success.

> changing sys.platform
on windows there are a number of different abi ( eg  msvc (32/64 supported) / cygwin / mingw / midipix  .... ) and one should expect sys.platform to point to the correct one.

The maintainers ( like steve.dower ) help choose carrefully those codes and ensure they will always work, so i think you have been answered.