The PR rating was a rating used creatively by Cyrix in order to better represent their earlier processors. For instance, the Cyrix PR 166 was not a 166mhz processor, it was a 150 (or maybe 133) but they gave it a PR rating of 166 because they maintained that the CPU had comparable performance to a Pentium 166. I am not sure if there is an actual benchmark that determines this rating (there must be, eh?). Intel Pentiums and Pentium IIs just happen to score a PR rating almost identical to their mhz. The same can be said for AMD K6/K6-2/K6-III. The fact that our Durons and Athlons (not sure about the PIIIs and P4s) PR rating is quite a bit higher than their mhz is due to the superior architectures in these processors that allow them to do more per clock cycle than earlier CPUs. I think they include PR rating in SANDRA for all us old [censored]s who remember the old days.
Hope this helps.