You are making a very good observation.
Actually, only one coupler opens when the car is unloaded. "Uncouple" fires both couplers.
This is a carry-over of the prewar versions that operated the same way and were identical electrically. The big difference is that the prewar couplers "re-closed" when the unload button was released... as they were operated(closed) by gravity. The train stayed together.
Which leads up to the early postwar development and eventual 1948 beginning of the roll-out of the magnetic coupler system still used today. The new system had to be completely compatible / reverse-compatible electrically with those prewar operating cars and all knuckle-couplered cars that had been built since 1945.
