Sumbitch they're supposed to shine a spotlight on the clouds to alert me when this comes up. It's in the shape of two people crapping in their hands and slinging it at one another.
It's because the hub bearings are seated on the axle and fixed to one another (like the front hubs on the front spindle) so the rotor runs true, compared to when there's a single bearing and the end of the axle is free to wobble within whatever play is in the receiving splines in the differential.