Leo beat me to it, but how to translate a 18x3 = Par 54 course into a regular average? Same for Par5s (Par 90). Is the former more difficult, the latter easier with all the eagle chances? To me, it's perfectly understandable that they stay out of such discussion by excluding them.
A 29 = 58 is about 4.5 from your average, for a 34=68 it's 5.5 => bigger difference, more change.
I doubt if the changes were not 0.02 and 0.03? Otherwise you'd have only approx. 50 RRs - your score history proves differently.
Until now, I've not found any faults in this calculation.