Advertized frame sizes can be misleading too since different manufacturers and even different models by a manufacturer use different relationships between the seat tube and top tube lengths. Once upon a time, they were all "square", meaning a 58cm seat tube got you a 58cm top tube. Not any more. I own two Treks (a 56cm Portland and a 58cm 1000) and they each have a 565mm top tube, as do my 56cm Schwinn Peloton and 57cm Litespeed Classic.
While the
actual measurement of the seat tube may vary, I've found that the advertised frame sizes are don't vary all that much even from company to company. For example, a 58 cm Trek may not a seat tube that is 58 cm nor will a Specialized nor Giant nor just about any flavor that you want to choose. But the effective top tube length of a "58 cm" bike is going to be very close in all the different brands and models...if the bike is designed for a similar use.
This makes sense too, if you consider that bikes are made for a fairly narrow range of body sizes and that some of those parameters are more easily adjusted than others.