Note that while both listed loads of black used 3 drams, the 3F load was pushing 1 oz shot while the 2F one was pushing 1 1/8 oz. The heavier shot load would account for part of the pressure increase along with the finer granulated powder. A third factor which can enter is they were taken at a different time & place so actual powder components can play a part as well. Quality of the charcoal used is a major factor in black Powder quality. I seriously doubt it is safe to say that switching from 2F to 3F will normally double the pressures if all else is equal.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra