Yep, it's the draw length. If you were to plot out the force draw curve graphs for my 50 pounds at 32" draw vs your 55 pounds at 28", you'd see that I'm actually storing a good bit more energy into the bow than you are, even though you're drawing more poundage. Also, my power stroke is longer, so I'm getting that stored energy powering my arrow longer than you. That's why I have to have the heavier spine arrows. I can't shoot 2018's unless I'm shooting a 40 pound bow.