I just thinked about thoise empty high ranked slots...maybe there happened something why lots of cats are missing (like in second book)? And when someone go and gets the empty ranks, he/she just make it look like that the cat was ecsaped whenever he/she was and come back to it's clan...
Just an stupid idea...again xD 

OR then thoise missing higt ranks are you know background cats, who are there and anyone can control them if needed...and when someone take the position...the Bg cat will just die...something else happen to them... or something like that...