We get lots and lots of very confused customers (and its easy to see why) regarding the stated sizes of tubular tyres (singles, tubs, glue ons) and to a lesser extent tyre sizing.
Basically and without going into reasons all you need to know is 27″, 28″ and 700c when refered to in TUBULAR sizing are all the same size (I.E. they all fit onto a standard modern tubular wheel).
The problam that exists if you are actually concerned as to why, is that standard tyre sizing refers to either a bead diameter (I.E. 622 this is also refered to as the ETRTO number) or
then there is the french measurment (the most common I.E. 700c) this refers to the inflated tyre diameter neither measurment are a physical wheel size.
I’m not going to go any further, space is limited and I dont want to confuse you, but here is a brief summary;
Tubular tyres: 27″, 28″ and 700c are the same. 650c and 26″ are the smaller triathlon size and will not fit standard wheels
Clincher Tyres 700c and 28″ are the same size. 650c are the smaller triathlon size and will not fit standard wheels. 26″ and 27″ are not compatible and refer to MTB and cruiser style wheels or very old road wheels.
One last thing 23mm is the optimum width for a road tyre, tubulars being rounder in aspect have a optimum around 22 mm.