that the hill is physically lighter than the pronghorn. a bow that's very light in the hand, can be harder to hold on target. too, the pronghorn is probably faster than the hill bow (at the same draw weight). so that might cause differences in in accuracy. the draw force curve is also a little different. the hill likely builds weight a little faster toward the end of the draw i.e., stacks a bit more. any one of these things can account for differences in accuracy. put them all together and it can be substantial.
it could be your hand placement on the grip, a hill style bow likes to have 