The measured height of a nominal size 56 CAAD 10, C-C, is around 54 cm. I don't think that's comparable to classic, horisontal top tube geometry, which is why the nominal size has nothing to do with the measurement of the seat tube. For what it's worth, CAAD 10 has a fairly aggressive geometry if you compare the top tube to head tube length (or reach to stack). If you compare the head tube length to older bikes, you have to take into account the headset stack difference - Cannondale uses integrated headsets that add around 5 mm to the headtube length, while the conventional threadless and threaded headsets add 20-30 mm.