It’s really silly that seat tube length is used as the single measure of frame size. Much more important is reach and standover height, since those can’t easily be changed. A longer or shorter stem will adjust reach at the expense of changing how responsive the steering is, whereas a short seat tube can be fixed with a longer seat post. Seat tube length is also measured differently by different manufacturers — sometimes it’s center to center (of the tubes), sometimes center to top, etc. Seat tube length is convenient but it’s not what people should focus on when trying to find a bike that fits.
It’s really silly that seat tube length is used as the single measure of frame size. Much more important is reach and standover height, since those can’t easily be changed. A longer or shorter stem will adjust reach at the expense of changing how responsive the steering is, whereas a short seat tube can be fixed with a longer seat post. Seat tube length is also measured differently by different manufacturers — sometimes it’s center to center (of the tubes), sometimes center to top, etc. Seat tube length is convenient but it’s not what people should focus on when trying to find a bike that fits.