Sorry what do you mean virtual seat tube I'm new to all this so I haven't a clue and have lots of thing to learn thanks
As ColinJ says^.
If you look at the 'geometry' diagram on the website the dotted line shows the 'virtual top tube', (but doesn't give a measurement for the
actual (sloping) top tube). On the other hand it gives the
actual seat tube measurement (53cm - which is what you measured), and it also gives a measurement for a 'conventional' seat tube - 56cm - this is what I call the 'virtual' seat tube - the measurement to the point where the dotted line would meet the seat tube if it was extended upwards. It would be really helpful if there was a universal convention regarding the way frames were measured and described. At the moment it can be really difficult to compare different makes with each other.