Some, especially if they only have one crested parent, chicks will not have a visible vault. They typically have a smaller crest. I have a half polish Bantam gal that I didntnjnow about until her little crest grew in. I thought for sure she was out of a different male
I've had a few chicks that look single at first glance when they're only a few days old. Their comb usually don't get as large period as the chicks that come out of the egg with a giant star smack in the middle of their face