The rose comb of your bird in the top picture is the normal comb for the Wyandotte. Some birds sold as Wyandottes, that appear to be Wyandottes in every other way, lack the Rose comb gene of the true Wyandotte. Occasionally breeders will dilute the pure Wyandottes intentionally, because the true rose comb Wyandottes have a lower fertility rate than the diluted Wyandottes. The genetics of this is very complicated and you can read about it at http://www.ncbi.nlm.nih.gov/pubmed/8072931, but the main point here is that, yes, you can sometimes get a Wyandotte that has a single comb.