Michael McIntosh has a good chapter on triggers ("A Toast to the Trigger") in his book, "Shotguns and Shooting". He refers to the "traditional rule" of optimum trigger pull being half the weight of the gun. But he also refers to a heavy target gun he owns with #3 pulls as feeling just right.
The reason he gives for the front trigger (he actually refers to "the first sear", which thus covers single triggers . . . except how do you know which sear is first if it's a selective trigger?) as being lighter than the rear trigger is that if your second sear is set too light, recoil from the first shot may result in the gun doubling.
