Falln: I agree that you're not changing the pivot point, but when you are changing the activation point, you are also changing where on the lever you are pushing down on. When the activation screw goes in or out, the trigger will travel more or less, depending on the screw length.
If you were to take a pendelum (bad sp) and have a marker at the end of the pendelum, have it swing and touch one flat surface, it makes a mark. Now if you were to back up the marker on the pendelum and swing again, it should hit a different point on the flat surface. With the change of length of the marker, the pendelum is swinging further. I hope that's clear enough, and I hope I didn't confuse this whole forum.