The sports books opened Stanford vs. USC with the Trojans favored by 4 points and has since moved to USC by 51⁄2. The change in the line reflects that early betters have laid more of their money down on USC to cover the spread.
Vegas lines and odds aren't meant to be predictive. They are made to entice betting in equal portions on each side of the spread. However, combining the line and the over/under usually ends up close to some computer models. After all, Vegas is very good at this and it is a lot of fun.