I attribute the penetration difficulty in turkeys to two main factors. First is the toughness of the feather quills. Tougher than deer hide and hair, IMO. Second, and probably more important, is the weight of the bird, compared to a deer for example. When an arrow strikes a turkey, it can and often does move the turkey a bit, pushes it so to speak. This definitely detracts from penetration.
In the dozen or so birds I've killed with a bow, on only two did the arrow zip right through the turkey. In all the rest, the arrow stayed in the turkey. The broadhead may have come out the other side on quite a few of them, but the arrow stayed with the bird.