Also remember the average 2" 20 gauge chamber measures 2" to the cone. Most 12 gauge so called "2"" chambers actually measure either 2 9/16" or 2 5/8" thus have a bit more clearance from the end of the loaded shell to the cone than does a 20. This clearance beyond the shell is what allows the crimp to unfold without a pressure rise. Since that "Additional 1,000 psi" with the longer hull was not consistent, but occasional, I have grave doubts that Bell ran sufficient tests to establish an absolute that it was the result of the shell length vs chamber & not just a normal variation from shell to shell. Within a given number of shells that much variation is certainly not unusual.
It has been "Said Many Times, Many Places" but I will say it once more "Never, Ever" fire a shell in which the "Loaded" shell end is pushed into the cone. This puts extra resistance on the crimp's unfolding & can create a rapid pressure rise.
