Except I thought dogs would score 21 more often than 20 when the team total was 20.5 and vice versa, and the reverse was proved true. The point is that we did find something higher than 52%, however. The followup is whether this is statistically significant.
I think the 20.5 example, if an exploitable finding, would be a symptom of a larger team total strategy. It's quite possible that something has been stumbled onto here. If so, this is just a minor attribute of it.
The next step perhaps would be to check 19.5 for a team total in the same fashion you have here, because 20 is the most common score (isn't it?). And then, I suppose, 16' and 17' because 17 is the 2nd-most common? Put another way, find the most common totals in the NFL and see whether anything weird has happened ATS for team totals.
While I am here, try not to put too much stock in team-based trends (not that we've been discussing those), because with the potential for there being no salary cap soon in the NFL, teams have made roster moves to take advantage of this. Similarly, assuming we do enter an era with a modified cap (or return to the era of no salary cap), most team-based trends will be pretty worthless on their own.