Interesting query about it being the frame tubing. But I have another hypothesis for the difference. Could it be slight variations of frame fit and geometry? Give us some information about yourself, your cycling inseam etc.. And then tell us what is the center to center size of the seat tube and the head tube of each bike. What stem length are you using. How comfortable are you on the faster bike. How does the slower bike feel comfort wise? Just curious since I have different road bikes of different sizes and have observed some dramatic differences in the average speeds attainable on identical courses dependent on which steed I use...I'm just sayin'