Read this http://forum.trackbase.net/threads/7...-rating-method
TSP is being calculated also with battle sense, engineering and how the others played, so if you had 100/20 in 'A' and you were the only one who had 17+ppm e.g. then you get easy +rate
but if e.g. 40% of the players in map 'B' have got 17+ppm then you probably won't get that easy a +rating anymore..
if the 40% are then just ppl with lowrating (let's say 4rating) then you won't get +rating bcs lowraters had the same ppm as a highrater so it's not good by the highrater..

hope you got it now , if you still have questions to TSP then ask,