Python is a language with one official supplier and many customers. Most of the other Pythons base their functionality in the official Python and alter the underlying code to suit the platform.

Browsers are supplied by at least three main sources and several smaller sources, each trying to convince everyone that theirs is the One True Browser by pushing forward at breakneck speed. There is never going to be total agreement (or even sanity) as long as that is the case. It's a mixed blessing, IMO.