That's the proper way to look at it...more margin. Team trends like New England generally starting slow are mostly meaningless. If Brady does sit for 4 games the math changes and the Dolphins are a natural beneficiary.
It reminds me of golf telecasts when a player might double bogey the final hole on Saturday and the announcers try to pretend it might help him, make him more motivated for Sunday. That's an idiot perspective. Ian Baker-Finch offers crap like that all the time. But on NBC Johnny Miller knows the applicable math. He'll say the guy's odds just went from 40% to 20%. It's remarkable how accurate he is, for someone with no background in speculation.
If the Patriots drop one game in season win over/under projection, as I expect, then Miami's division odds move closer to New England. Fairly basic. The Patriots might overcome it in a one-season 2015 sample but if you played it out repeatedly with Brady missing the first 4 games they wouldn't win the division or make the playoffs nearly as frequently as if he played all 16 games. Big picture.