Good point, the same happens in our Open. (You know - "The Open", not the British Open or whatever people over there call it ......... just kidding obviously :) )
I mentioned Justin Rose - as an Amateur off plus 3 he didn't only make the cut, he tied for 4th place in The (British) Open. Ok this is a tad 'unusual' and he then kinda struggled once he became a pro for the first year but even he says that was purely psychological.
The pros are on average about 5 shots a round better than a scratch player. So lets call it over 18, 1 less putts, a couple more up and downs, hit the green in reg one more time and hit the fairway one more time (apart from at Riviera where it made *&^%*& all difference.....) That's the difference. it's a lot at that level, but that's because scratch players are already pretty good.