The K3451/53 are comfort height and use Kohler's class 5 flush technology.
The comfort heigt bowl adds to the price in any brand. The class 5 is their version of the large diameter flush valve. This is the technology introduced by Toto, and now widely copied, as in AS Champion, etc. This type of technology also jacks up the price in any brand. Kohlers are pricey because of the "Stately" or "Classic" design.