Intrigued, I ran a few Quickload simulations using loads and powders I have used. The first few QL runs showed very similar results with QL's prediction being about 8% higher peak pressure than Boyles law. Here's one such example (UCV=Usable Case Volume in H2O grains):

9mm, 115gr Rainer RN, HP38 4.7gr:
-- COL=1.169, Pmax=24781, UCV= 10.027
-- COL=1.100, Pmax=32440, UCV= 8.301
-- Boyles Law = 24781x10.027/8.301=29933

So "actual" pressure in this and other examples increased ~8% more than Boyles Law predicted.

Then I decided to change powders, and got a distinctly different result:

9mm, 115gr Rainer RN, TiteGroup 4.7gr:
-- COL=1.169, Pmax=19479, UCV= 10.027
-- COL=1.100, Pmax=41486, UCV= 8.301
-- Boyles Law= 19479*10.027/8.301=29529

QL's predicted Pmax is 76.3% higher than Boyle's law would project. (Be carefuller with TiteGroup lol).

So as hdwhit said . . . there's other stuff going on here.