The type system is specified but the main pep people point to is the base type system. There have been multiple extension peps adding new type features and different type checkers have different progress implementing them. Also different type checkers have different bonus features. Mypy comes with plug-in support to make it easier to extend the system for very dynamic custom situations. Pyright is able to generate rough guess type stubs for untyped codebases. Pyre has the best tensor related type support likely motivated by Facebook having pytorch. Pytype I know the least about but it likely has some unique features motivated by google usage.

If the type checkers disagree on stuff defined in pep 484 that’s a bug to report. Beyond that they can vary but feel mostly similar.