Probably Specialized and Trek are measuring from the BB center to a imaginary horizontal line coming from the top of the head tube (or there abouts) intersecting the seat post.

Ridley might be just measuring to the intersection of the top tube with the seat tube as we did back in the days when most all bikes had a horizontal top tube.

Sizing varies even among different models from the same manufacturer. Don't assume because a 56 cm Émonda SLR 7 is recomended for your size, that a 56 cm in another model Trek is correct for you.