That is a wonderful chart!The comb is controlled by 2 gens that any of them have two alleles("option") and any of them must be present in 2 copys in the. Chicken, one from the roo, and one from the hen.
The gene R- rose comb that can be ominant-R , or recessive-r
The gene P- pea comb that can be ominant-P, or recessive-p
The combination of gens are:
RRPP- walnut comb
RrPp-walnutcomb
RRpp- rose comb
Rrpp-rose comb
rrPP- pea comb
rrPp- pea comb
rrpp- single comb
So the parents of your chicken aren't homozygous for the comb genes!