Yep. What McDave said. The draw force curve is usually a little different between the two types of bows, with longbows tending to build weight a tad more toward the end of the draw. That may make them feel heavier, but ultimately, if they're both 55#, you're still holding the same weight at full draw.
It's possible, too, that the poundage on one or both of the bows you're shooting is mismarked. That's a fairly common occurrence.
Lots of other design and composition factors also come into play. Some bows just stack more than others. Shorter bows tend to stack more quickly than longer bows, for example, but there are all kinds of things that can be done with bow design and materials to smooth out the draw force curve regardless of bow type.