I believe it was so the coupler pocket cover could be snapped in place and clear the bolster hole ring while doing so. The original Kadee-made cast 50' underframes used to allow for the offset, but I don't believe the current versions do. What's interesting is that the medium-extended coupler pockets do not have the offset, last time I looked. Also interesting is that the bolsters on many of the MTL models don't align with the side bolster detail on most of the models regardless of the truck bolster being used. On some of the models, you would have to body-mount couplers and rotate the offset truck 180 degrees to get everything aligned.