Honestly, i don't fully understand the process either, but it's normal operating procedure with every auto manufacturer i have ever dealt with. Everything comes out of a PDC (parts distribution center), very rarely do you ever get something ship from source or ship direct. In this case, i know the differentials are assembled in Mexico (though they look like a Getrag design), i believe they are then shipped to Valmet for installation with the electric motors. I would hazard a guess that they goto detroit in a quanity greater than one at a time.
Also, that picture it is about 500LBs worth of metal. the electric motors are 200lbs each. With the torque output of the motors, you need a lot more strenght.