Assuming the traditional sizing standards of seat tubes from center of BB to center or top of top tube, the differences you see are because MTB/hybrid sizing allows for more top tube clearance than road bikes. In traditional sizing, I ride a 19" (48.3cm) MTB and a 58cm (22.8") road bike. Nowdays, with sloping top tubes and such, there are so many different ways bikes are measured that it is hard to tell the size of a bike without comparing several different measurements. You can't be sure if it is an actual measurement or a virtual measurement.