Can someone please answer this question. I am looking at a couple of bikes online that both have the same size wheels. One is a 21.5" frame; the other a 23" frame. Both say the stand over height is 32". How can that be? BB height difference? The 21.5" bike does have larger tires, but by less than a quarter inch. I contacted the sellers and confirmed the measurement as being the exact measurement from ground to top of cross bar. Both have level horizontal top bars. Can the top bars be placed in a lower position on some frames?