The II models have a firing pin safety, the type that allows the hammer to hit the firing pin after it has been deactivated.
In a Colt 80 Series gun, the safety is deativated when the trigger is pulled completely to the rear. That's why a lot of people prefer the 70 Series guns; the claim is the 80 Series safety adversely affects the trigger pull.
Kimber is using a similar system, but it is deactivated by depressing the grip safety and since it has nothing to do with the trigger, is said not to affect the trigger pull in any way.