Could the difference be based on the potential drawbar pull applied?
These locomotives were operated hood forward, and the more substantial coupler is at the rear of the locomotive. It appears to me that the front coupler could absorb a lot of pushing force, but not nearly as much pulling force.
If a significant cut of cars was to be moved any distance they would be at the rear of the locomotive where the coupler could withstand the locomotives full tractive force.
Only a supposition based on looking at the hardware.
Rex Beistle
NMRA RMR
Estes Valley Division