For some reason different manufacturers measure their frames using different standards. Even on conventional frames with level top tubes this has been a problem. With some manufacturers they measure from the center of the crank to the top of the seat tube, some measure from the center of the crank to the top of the top tube, and others use the center of the crank to the center of the seat tube.

With a compact frame, I look for the standover height, AND the effective seat tube height. The effective seat tube height is the height that the seat tube would be if the top tube was parallel from the head tube to the seat tube.

While I don't own any right now, I have been thinking about trying a compact frame to see if I can improve on my standover height probem, since I have short legs and a long torso. 56cm, frames usually have the top tube length I need, but the standover is a bit long. 54cm frames usually have a good standover for me, but the top tube can be too short. I'm thinking I need to try some compact 56cm, which should get me into a good length top top at a lower standover height.