Rather than explaining it another way
From Sheldon Brown:
Measuring Frame Size
Generally, when you see a single number listed as a frame's "size" that number refers to the length of the seat tube .
A further complication is that nobody knows how to measure a bicycle's seat tube any more. Even leaving the inches/centimeters question out of things, there is the question of where the seat tube ends.
* The old standard system was to measure from the center of the bottom bracket to the very top of the seat tube.
* Some manufacturers have decided that this is too easy, so now many bikes are measured instead to the intersection of the centerline of the top tube with the centerline of the seat tube.
* Some other bikes that have seat tubes that protrude farther than normal above the top tube measure as if they were measuring to the to the top of a seat tube with normal protrusion.
* Some bikes are measured to the top edge of the top tube, even though the seat tube protrudes higher up.
* Some bikes with slanting top tubes are measured as if there were a level top tube, they use the length that the seat tube would be if it was as high as the head tube.
Anarchy reigns; I know of one bicycle line that made a running change in the middle of the year. You could have two bikes of the same make, model, year and nominal size, but one was 2 cm larger than the other! The only way to know was to measure them.