The ratings are always done on a per hole basis and then expanded to the number of holes on the course. The impact of one throw on the ratings is related to the total number of holes in the same way it's done with any statistics. Think of it this way. Consider that you played two 18-hole rounds and shot 50 on each one rated at 1000 each. If you shot one better in the first round, your rating would have been 1010 for that 18-hole round. But your average for 36 holes would only have gone up 5 points to 1005. So one throw affected your 18-hole rating by 10 points but your average rating for 36 by only 5 points. The more holes, the less points per throw for the same SSA caliber course.
Your idea that a throw value should change as it moves away from average makes sense from a probablity standpoint for a specific player but which player? Your average round is 1000 rated but for me I might hit a 1000 rated round one in 30 rounds. Should I get a better rating for shooting the same score that you average regularly? No. That's why the rating points per throw for a specific SSA course with a specific number of holes is a constant. Every score is "average" for some player even those who may not exist yet (1060 average player).
Rater of the tossed arc.