an rear exhaust design is much more space restriced. there is a very limited grille thru which to dump the hot air. yb blowing the air into the case, you can use a bigger fan, and move more air per minute. obviously though, this method requires good case ariflow. when used in a case with good airflow, it is the more efficient method.
some cards use an inbetween, dumping some air into the case and some theu an exhaust.
most aftermarket coolers dump into the case too. they can use large fans, and the sort of people who buy them will almost certainly have good cases with lots of fans. so they can get much better temp decreases.