IOW it's off by an average of zero. The standard deviation is 13.45. I tested this on NCAA games. The mean is the same and the standard deviation is actually closer to 16.

Using previous research by Hal Stern[1], Winston posited that the final margin of victory for an NFL team in a given game can be approximated as a normal random variable with a mean of the Vegas line and a standard deviation between 13-14. (Winston and Stern’s exact number, derived from the 1981, 1983, & 1984 regular seasons, was 13.86; I’m using 13.45, based on the overall NFL average from 1978-2012.)