The PAX does not assume that that AM stays at the same speed. AM was getting faster until the past few years. Now. the BBR Shark had no real challengers and development in AM has essentially ceased. The graphs show most classes getting slower for a period of time before turning upward again. One major reason for this reversal is the recent rapid developments in R comps and street tires. The upward trend started in 2006 so it is based on 2005 results which was the last year at Forbes. But, most people are surprised to learn that Rick does NOT use the Nats in the computation of the PAX.
The Nats Results for 2005 would have been really useless. There was the thunderstorm Tuesday morning followed by a major change to the north course because it broke up and then Stagnaro oiling down the north course. A CM oiled down the south course on Wednesday and there was more rain on Thursday.
Wasn't the stock category realigned early in the decade. The graph seems to indicate that. The relatively flat curves for BM, CM and FM are due to the cars being GCR legal which limits how much they can be developed. I believe DM and EM have had weight added so that will slow them some. Also, in both Prepared and Modified, Hoosier has little competition so that slows tire development unlike in the other categories.