A spread has more do with than what Vegas believes the point difference will be between the two teams. Just because UConn is -2.5 does not mean Vegas believes they will win by 2-3 points. Vegas wants to even out the action as best they can, for they don't get burned either way.
For instance, when UConn played UK in the Final Four 2 years ago, UK was -2.5. Even though UConn was the lower seed, one of the hottest teams in the country (9 game winning streak), absolutley destoryed them earlier in the season, and overall the better team. Vegas was probably aware of other factors such as UK's huge following, and national brand (even more-so than UCon). They wanted to even out actions and entice other gambles who woud have gone with UK +1 but since the spread was at UK -2.5, decided to go with UConn. This ultimately evens out that actions and why you see the spread for the Super Bower move up and down so much when it is first released.
Anyways, lets go UConn girls!