Frame sizing and frame measuring are really two different things. Generally, a frame measurement is the distance from the center of the bottom bracket to the top of the seat tube. This used to work well when all bikes had the same shape: a standard two-triangle concept. Aluminum and carbon fiber have made various tube shapes possible so that top tubes often slope down towards the rear axle now. Imagine your Cannondale frame and just moving the back of the top tube down two inches, but lengthening it so that your "effective top tube" measurement is the same. Your 56cm frame is now a 54cm frame, even though your saddle height wouldn't change (it'd just stick out of the seat tube more) and your effective top tube wouldn't change (at least not much).

I have two Treks that I ride. One is a 1997 750 with a steel frame and a straight top tube. It does angle down SLIGHTLY, but it's more or less a straight top tube. It is classified by Trek as a 21" frame, and the center of the bottom bracket to the top of the seat tube (TUBE, not POST) measurement is exactly 21". This is often called the C-to-T measurement, or Center-to-Top measurement. My other Trek is a 2015 Verve with an aluminum frame and a sloped top tube. The whole frame has a different geometry because of the suspension fork (so the head tube is much farther from the wheel hub). It's listed as a 20" frame, and the C-to-T measurement is, indeed, exactly 20". However, it's a larger frame than my 750, despite the smaller nominal number. The effective top tube on the Verve is a little bit longer than on the 750. And if you took the Verve's top tube and imagined it to be straight back from the head tube (or nearly so), the C-to-T measurement would actually be a number of inches LONGER than what it really is.

So, although frame measuring itself often uses the C-to-T method, frame SIZING doesn't necessarily correlate with that number. That is, two 20" frames might fit you very differently, depending on various other measurements of the frame.

I get Giant's idea of just calling them "M", "L", etc, because a single number doesn't always tell you what you need to know. By the same token, though, a single letter doesn't, either.