Bike have top tubes appropriate for their size. We're just saying that people size down to get the associated shorter head tubes to be able to get a lower position. I picked the smaller of two sizes that would have worked for me mainly to get the bars as low as possible, and also got the special "team geometry" version with an even lower head tube than normally found on that size. (Althought o be fair it only restored the head tube heights from prior years before jacking them up )