Mostly because their division is probably the best in baseball. The Angels are going to get better as the season progresses. The Mariners are likely to get better. The A's are playing well--there are just no gimmes in that division.
The other issue is whether guys like Vogt and Burns can continue to produce like they have. No one in baseball expected Vogt to become a catching version of Jason Giambi when the A's picked him up from the Rays. Burns has never produced in the minors like he's producing now. You have to expect some regression. Then again, no one predicted that Josh Donaldson would become one of the best players in baseball. Maybe Beane just has an eye for guys that exceed expectations once they hit the majors.