http://www.pokerstars.com/poker/room/rake/
I'm looking at the rake page right now and it says that if there are 2 players in the hand at 2/4 and higher the rake is $0.50 at $20 and then $1 at $40.
Perhaps it's different for the heads-up tables and they didn't update their rake page?
Either that or it appears you are mistaken.
I just saw that on pot-limit and no-limit for 2 players in a hand the rake is $0.05 for each $1 in the pot up to a max of $1.
I didn't know the rake was taken at every $0.05 on NL tables even at higher stakes so this would probably have the effect of making your rakeback even worse than I had previously speculated.
Anytime you get a $35 pot or something then you'll have higher than a $0.50 rake of course.