I wonder if there is also a difference between breeds. My 5 breeds are feathering in at very different rates. The welsumers go at lightning speed, at 2 weeks their wing feathers are almost totally in and their tail feathers are growing rapidly too. On the other hand the Light Brahma's hardly have any wingfeathers yet at all, and no tail to speak of. The other breeds (australorps/dominiques/GLWs) are inbetween these two extremes. They have a decent amount of wingfeathers and a little bit of a tail to come through.
I don't think that all three of the light brahmas are boys. That'd be too much of a sexing error at the hatchery. So perhaps it is also breed dependent.