The way newer road bikes are sized can be confusing (I have been learning this a lot as I shop for a new frame). Most old bikes did have a horizontal top tube and now a lot have sloping tubes and compact geometry which means that the actual top tube length can be deceiving and you must go by effective top tube or virtual top tube. Also a lot of companies frame size has nothing to do with the top tube or seat tube measurement. I just read a good article about this (of course I didn't bookmark and can't find it now). But they pointed out how one bike's size was 50 but the top tube was 545mm and seat tube was 520 so it was really like a traditional 54/55 size bike. So yes, sizes are different and if you are shopping at an LBS let them size you and suggest what size to buy