This is not how you should be calculating the average point difference per game. You're letting negative and positives differentials offset each other. You should be using the absolute value of the differentials. For those ten games you get a sum of 144. So the average point difference was 14.4 points. This is only the average point differential and doesn't indicate who it was in favor of. But it does indicate the Vegas's spread was, on average, off by about 2 touchdowns. Which, in my opinion, seems pretty significant.
If that was my goal, that's what I would have done. But it wasn't. It was to get an idea of how over or under-rated (or over- or under-performing in the bowl games, as a whole) the SEC was. Yes, Vegas spreads are always off because of variance, that's how it works. My goal was to see if they were consistently off by a meaningful margin in one particular direction.
I just noticed you had 9 games and not 10, so the average was really 16. This indicates the magnitude of how far the vegas spread was off on average.
I don't believe you have achieved what you wanted with your calculations. A weighted average may be appropriate here to minimize the effect of outlier games. You have 5 positive differentials with a sum of 248 and 4 negative ones with a sum of -82. So 5/9 * 248 + 4/9 * -82 = (1240 -328) / 9 = 101.33 for the weighted point differential sum. The average would then be about 11.259.
Edited for clarity (misstated intent of weighted average initially)
Ugh. Have too much crap in my text editor scrap page. So 5/9 * 62 + 4/9 * -82 = (310 - 328) /9 = -18/9. So actually, an average of -2. So, yes, your calculation is close anyway in this scenario.
So, in terms of just over or under predicting, Vegas seems pretty fair. However, in terms of how accurate they were, they seem pretty off. On the one hand, I guess you could say that SEC represented themselves, fairly well, with respect to the spread. However, the spread had pretty low predictive power (seen by looking at the average magnitude of the differentials) so unless you were interested in just betting, and then only on the whole range of games, I don't think it was too indicative of the SEC's strength.
Edit: fixed minor typo
Edit 2: fixed redundancies in sentence