Those are the measurements for the nominal sizes 54 and 56. It seems to me that you had a mis-marked frame. Top tube of 57.5 cm has meant nominal size 58 for Cannondale for the last decade at least - just check the geometry charts.

Personally, I find the Cannondale geometry charts to be exceptionally clear and informative, but I guess the abundance of information can make them confusing for those who aren't as interested in every little measurement there is.