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@Oswald
ST = seat tube length from the centre of the bottom bracket to the centre of the top tube.
TT = top tube length from the centre of the seat tube to the centre of the head tube.
Your Specialized is 54cm .. If the frame had a horizontal TT this would be the ST length. I think if you measured the TT from the same virtual intersection point it is also 54cm. So the Principa is the same ST and 1cm longer TT. I hope that helps. Also maybe look at a frame geometry drawing on Specialized web site, that may make it easy to understand if you're still in doubt. Beware though because not every manufacturer lists frames in the way .. Colnago being one In particular.