Second, those spreads move as individual bettors decide who they like. Which again, isn't always in lockstep with what poll voters think.

In the ESPN FPI, BYU is roughly a 2.3 point favorite on a neutral field. BYU isn't on a neutral field. Furthermore, it's entirely possible that Vegas makes a larger than usual home field adjustment when a team is traveling 2,000+ miles to play the game.

Now I'm not saying that Vegas is using the FPI to set their initial spreads - they almost certainly use their own proprietary ratings system.

Is it ridiculous that BYU is a very slight underdog? Not especially. In fact, there are several places which still list BYU as a favorite.