There is another possiblity here. I don't pay a whole lot of attention to what the other teams defense is doing. If they're playing zone cover 2s (or zone cover whatever) on every play and dropping back the LBs, while maybe even pulling DLs and moving them all around to pressure the QB, then DDs rushing yardage is much less impressive. If they're using cover one, man-on-man covers on every play and letting the LBs stay in place and just watch the ball, then DDs rushing yardage is much more impressive.
I noticed that our D used a lot of 2 DL defenses today and A LOT of DBs to try and stop the passing, but they still got some pressure on the QB and mananged to STOP THE RUN. I was impressed. I've no doubt they were dropping the LBs back most of the time too, but they STILL stopped the run pretty well.