I think this might be happening because of the orientation of the servos. So the rear diff needs its servo to be fully counter clockwise to be locked, where the front diff needs its servo to be fully clockwise to be locked (I may be mixing up locked/unlocked here). So, when you switch the channels over you’re inverting the operation as well as changing the channels round.
That sounds confusing trying to explain it without going into too much detail. Almost needs some sort of graphics to explain properly. This view might help though, when you consider the cable is being pulled/pushed in a different direction (clockwise/counterclockwise) for the front and rear servos.