The universal frame sizing system is woefully out of date. The only time it really made sense was after most quality manufacturers adopted Eddy Mercxes' 'square' frame design (where the top tube length matched the seat tube length). Now that compact frame have been thrown into the mix it's become a minefield. How this for lunacy? We did this on Spandexes Paddy Wagon recently. To establish the frame size we had to measure from the centre of the bottom bracket to a horizontal line projected from the top of the head-tube. ie measuring to a point in mid air as far as the frame itself is concerned.
Frames are described using the seat tube dimension, which is area of the bike with the most adjustment, on a mountain bike for example the range of adjustment can be eight inches or more. The length of the seat tube is totally irrelevant.
The critical dimension is reach, a combination of top tube length and stem length, I buy my bikes for their top tube dimension, it matters not where the top tube is as long as I have enough stand-over clearance, whether that's two inches or twenty two inches doesn't matter so long as the frame is designed to accept a seatpost long enough to reach my saddle.