Thats because the 1911's are a single action, so the trigger/sear relationship is constant. The double action autoloaders also provide the ability to cock the hammer with the trigger (double action) as well as single action, and as such, have different engagement points to accomplish this. If you're still curious, try stripping a 1911 frame and one of the others you mention, and you will easily see for yourself!
P.S. I would have someone on hand who knows how they go back together-especially the DA autos!