When there are multiple boys often one does not show himself until later. They mature more slowly and don't crow as young. That plus this boy being white made his saddles less noticeable.
Sometimes boys are sneaky!
I think you'll find that one of the hens laid the egg. ;)
He clearly has saddle feathers in these photos as well. Sometimes saddle feathers are harder to notice when they are the same shade as the body feathers. Maybe they were there all along and you just didn't notice? :)
Do you have other roosters?
Did you or a broody hatch him? Other roosters...