Before commit 8334f30a74abcf7e469b901afc307887aa85a888, at the first _imp.create_builtin() calls, it calls _PyImport_FixupExtensionObject() which stores the created module in an internal "extensions" dictionary. PyInit__weakref() returns a module object. Next calls to _imp.create_builtin("_weakref") simply returned the cached _weakref module.

At commit 8334f30a74abcf7e469b901afc307887aa85a888, _imp.create_builtin() doesn't store it in the internal "extensions" dictionary, but calls PyModule_FromDefAndSpec() instead. PyInit__weakref() returns a module definition (PyModuleDef_Type: "moduledef"). Each _imp.create_builtin("_weakref") creates a new module.