Not all of the PM6100s will OC to 90 MHz. At least that was the summary of the reports from back in the day. There were newer adn older revisions of the PPC601 chip and that plays a part in how much OC the machine will tolerate.
However, I also suspect that the voltage regulator may play a part. Apple used an adjustable LT1086 and the voltage level is set by the relationship of two resistors. Resistors have tolerances.
I suspect that if you get a PM6100 with the resistors at one end of the tolerance range, your PPC is getting a little higher voltage and the thing overclocks better. Get a PM6100 with the resistors at the other end, and the voltage is lower and it does not OC so well.
But that's just a hypothesis.