[mhespenheide]

Before mountain bikes and the Giant TCR, essentially all road bikes had horizontal top tubes. In that era, the size of a bike was referred to as the length of its seat tube. That wasn't perfect -- because some people measured center-to-center, others center-to-top, and also because different seat tube angles interacted with different top tube lengths. But it was good enough in most cases to get you in the right neighborhood, and you'd go with a slightly longer or shorter stem and live with it.

After sloping top tubes and long seat posts started to proliferate, the (effective) top tube length began to be more recognized as the better single number defining fit. Still, though, that discounts the effect of different seat tube angles and head tube lengths.

So some people started to use stack and reach as a better system, but they do need to be used as a pair of numbers and not a single number.

So: there's no good single number that really works. Some people are stuck in the deeper past of "what size seat tube would this be if it were a horizontal top tube". Some people are a little farther ahead and are describing "what top tube would this be if it were a horizontal top tube". Bike geeks like those on this forum want to see the entire geometry sheet. Someone like Mike Varley figures it's all a mess and no single number really works, so why not refer to a frame size as actual seat tube length and figures that his customers with looks at the entire geometry.

¯\_(ツ)_/¯