Also, part of the killing of the master was not just to show the apprentice had increased in power, but also as a way to cull masters who'd grown complacent or weakened, secure in their position. Sith needed drive, so once a master stops pushing, that should give an apprentice an opportunity to kill them even if the apprentice isn't strictly stronger.
Originally posted by jmoul
Although, by the Rule of Two, every new master was more powerful than the last and the new apprentice had to have the strength to surpass them. Maul was the apprentice after nearly a millennium of the Sith using the Rule of Two. So, unless he was a filler apprentice until Vader was ready to turn, wouldn't he technically stronger than Zannah in the other aspects?
Only if and when he got to the point he'd beat Sidious.
I'm sure there's plenty of failed apprentices that never get to that point.
Zannah completed her apprenticeship. Maul never did and was still some distance from it.