Because in the tests your function is called once and twice: rot13(rot13("some_string")), and your function isn't returning, so the default returned value is None, the result of calling your function once, and that's the input of the tests where it's called twice. Not a kata issue.

rot13 on predefined strings
Log
test <- input of the test, a string
None should equal 'grfg'
^
Your function returned None
Log
test <- input of the test
None <- your function returned None
None should equal 'test'
^
Your function should have returned the original string when encoded twice