I'll bite. 32% meeting the Boston standard is a cumulative effect.
1. CIM is not a destination race, nobody goes to "scenic" Folsom to finish in Sacramento. People show up to run fast.
2. The weather is consistently favorable, little wind, temperatures in the 40s to low 50s, infrequent rain.
3. A wealth of talented runners. Threads like this are free publicity that in turns attracts more runners seeking to run fast.
4. 2017 and 2018 were USATF championship years, guaranteeing a large(r) elite field.
5. NET drop of 320' While it descends closer to 1000' feet, there's about 680' of climb.
At this year's USATF meeting, which was the same weekend as CIM, the board wanted to eliminate the race as a valid qualifier for national teams. Because it was the American championship, the motion was revised to keep CIM included for qualifying purposes, but the line was drawn for anything with a greater percentage of decline than CIM (roughly .23% ) to be discounted for the purpose of making a national team.
I'm all for eliminating Revel-style courses, but what happened was there were too many Boston qualifiers, so they simply revised the qualifying times to narrow the field to the desired size.