I always measure C to C. Not sure 8f it's a better standard, but I do remember that most manufacturer's catalogues list frame sizes that way.....
You have to remember that the height of the top tube from the ground can also be affected by the geometry of the frame. Some frames have BBs that are higher off the ground than others, which affects th height of everything above it accordingly. I assume that seat tube angle could also affect tube lengths ant height in some way. It's not an exact science, fitting a bike to a person. It changes a bit from frame to frame..
Case in point, I have a 51.5 cm Bottecchia that everyone, including myself, at first glance could swear should be a 53 or 54cm. But measuring the seat tube (C to C) something like 10 times already, it still measures 51.5cm everytime.....
Our forum's resident Bot expert explained that it is all due to the frames geometry.