Originally Posted by
corrado33
There's no way that one of those bikes is a 63 and the other is a 61. There's far more than 2 cm of difference between them. Is it possible that they were measured differently?
There are different sizing standards.
Geometry from the BMC website lists no particular measurement of 61cm for the frame labeled "61", closest is 621mm 'stack' which is height difference from center of bottom bracket to top of head tube. Seat tube itself is 550 center-to-clamp, so probably more like 53cm center-to-center, which explains the long seatpost.