Those dudes in Vegas are rich for a reason, the deck is stacked...
Stacked in the realm of reality.
Picking Miami to have 7.5 wins says most betting people don't believe Miami can duplicate or even reach last year's record. The odds are set so that Vegas wins. If most people think the line is wrong, it will move up or down.
Miami had a negative point differential last season, it is exceedingly rare for a NFL team to have a winning record when they score less than their opponents. Miami had a 1-5 record against teams with a winning record last season.