I'd suspect they'd factor in things such as with the Pats being a veteran team, its unlikely the team would put any rookie in a position where he'd be noticed as a rookie of the year.
I'd also suspect that from a positional standpoint, RBs, WRs and sometimes QB rookies can have big numbers. A safety or CB making a high number of tackles doesn't get the same attention 1500 yards rushing or receiving does or 4000 yards passing might, even if the defensive play was critical to a team's success.
All that being said though, given the Samuel situation and potential DB injuries Meriweather could wind up getting a lot of playing time - but the coaches clearly would prefer to keep him in a nickle role at least for awhile - but it still could be worth a good bet!