When I buy chicks at the feed store that is straight run i buy the ones with the longest wing feathers. Reason being, females have the two defined rows at that early age whereas males have one. Therefore at 4-5 days old there is a good difference. Genetics can affect them as well as individual issues. But like on your birds, one is quite feathery and one is behind. For me, that usually is the pullets that do well with feathering a little quicker. Just my opinion though as a chickenaholic who buys way too many baby chicks..