That is because the two traits are located very close on the same chromosome. A cushion comb is a type of walnut comb. It is usually found in combination with single comb, forming a duplex single comb. I have one hen with a duplex rose comb. In the front it looks like a small buttercup, but merges halfway back into a rose comb just like a Wyandotte's.
Lowercase letters are recessive.
A blank means anything could go in that place because whatever is in front of the blank is completely dominant. They will have a tall three-ridge comb kind of half way between a pea and single comb. Thanks Uncle Buddy for your response. We have had pea comb roosters with big floppy combs and others with more compact combs so that explains that phenomenon. I understand from your explanation that R-PP would result in walnut combs.