Most frame sizes are measured centre of the BB to centre of the top tube, or centre of the BB to top of seat tube. It would be nice if the measurements when it comes to a, m, l or XL were just one of the two across all makes but some use one, some use the other.
Ultimately the size is whatever they choose to publish, you've got to work out whether it's right for you. To add to the challenge, top tube lengths aren't the same so it's a process of getting the vital measurements and working out if they are what works for you or not.
I spent most of my life thinking I rode an 18 inch frame and that it was right for me, whatever the brand, but eventually came to the conclusion that 18 didn't mean 18 all the time, some frames were too small, so after a bit of trial and error, found that actually, a frame that was 18-19 c2c was what I needed, certainly not 18 c2t, which are too small.