It has nothing to do with poor design. The correction factor is a prop constant that tells you how good an air brake that prop is for static thrust measurements, when efficiency is zero per difinition. The speed charts of the props are a good guess, which in most cases is better than a bad calculation.
ALL props that look anywhere near decent are about 82 to 85% efficient if allowed to fly at their design speed. Even so the Wright Brothers prop.
I know the APC prop makes more speed, but that is due to the larger true pitch than noted on the prop. The larger than noted pitch increases the prop constant in my spread sheet calculator, but does not increase flight speed, which is based on true pitch. I would need wind tunnel test results or fixed rpm caroussel tests with all the listed props in order to put in a pitch correction factor as well. It can be done, but not in my available time frame.