Even with the same parents... my sibling don't have the same body weight, height, or hair color... their are still variations that occur because that's the nature of it.Genetics isn't much of a factor considering that all the chicks I raise have the same parentage. The difference has not been genetics, but heat source. All other aspects have been identical. Brooder is the same, location is the same, parentage is the same, feed is the same.
I respect what you are saying is YOUR personal experience though.