It's pretty common for chicks to be put in the wrong bins and the labels mixed up. If you post some pictures of your other chicks, we can double check their breeds for you.
It isn't a Cornish X. It looked mostly white to me in the other pictures you posted but I can see now that it is buff. It could be a buff Orpington, or it might be one of the hatchery meaties NatJ was talking about in the post above mine. Time will tell.