Only Kansas City and Baltimore are better AFC teams than Buffalo. The Bills are several levels higher than the Raiders especially with Las Vegas dealing with a cluster injury problem at wide receiver and in its offensive line. 
The spread is short because the Bills are traveling cross-country fresh off a victory while drawing the Raiders in an angry mood following their loss to the Patriots.
Don't overthink these situational factors, though. Buffalo is far better than Las Vegas on both sides of the ball with Josh Allen developing into an elite force in his third season. Allen already has set a Buffalo team record by accounting for a dozen TD's through three games. 
The Bills are healthy again at linebacker. Their defensive line has tremendous depth and their secondary is very good. Derek Carr could be down two starting offensive linemen and three wide receivers. Buffalo is giving up just 17.2 points in its last 10 away matchups. The Bills also have covered 78 percent of their past 11 road contests.