It's my understanding that the 50° and higher seat angles, give better high lift flow, at the expense of low lift. Some porters, at least, in part, do that to keep from loosing as much of the intake charge out the Ex with large over lap's. Some use 40° seats to encourage low lift flow, also.
Besides the 32° & 20°, top cuts, I've also seen 27° or 28°, too.
I've studied aerodynamics and from that stand point, you don't want angles of more than 12°, in general, as that's where stall starts.