As I said in the other thread. They were always set to be about nominal size. Nobody much stated an official number until Continental got tired of people holding the numbers hostage so they computed what they decided would be an acceptable leakdown to them (or at least stop getting people complaining about the quality of their product). Of course, I have my own beef with Continental over other issues, but this one is much ado about nothing.

It's the same as why we use 80 pounds. It was a good number for round engines because it gave a reasonable check of the seal without making it too hard to hold the piston at tdc.