The dog on the run line correlates with Unders, the favorite correlates with Overs. For example it's much easier to cash with the favorite on the run line when that favorite scores 8 runs as opposed to scoring 4, and the Over is going to cash far more often when the favorite is scoring 8....
It's a big edge for bettors, take advantage of William Hill's opportunity while it lasts -- there's a reason that the vast majority of sports books won't take RL/total parlays in MLB any more.....