a number of factors, the very least of which is how good 'they' actually think the two teams are (ie, they know the last time we played them in NC we were embarrassed and that this will influence more money going Bama way, so they make line higher to discourage money from going that way).
Having said that, to my eyes Bama looks clearly better than us (more glaring to the 'eye test' than going into NC in '12) but, how many common opponents have we faced? Sometimes the eye test can be misleading if they are no common benchmarks, and that's what we need to hope the case is IF we get past Clemson.
But I wouldn't say we are outclassed by them any more than Clemson, Georgia, Tosu, OU, or michigan would be.