The A's have 64 games left to play. The Angels have 65 games left to play. Let's eliminate the common games and see where the remaining schedules differ.
The teams will play each other 10 times. Each team has 4 games left against the Twins and 3 left against the Rays. The A's play the Rangers 1 more time (10) than do the Angels (9), and they play the Phillies one extra times, too. The Angels have an extra game remaining with the Mariners. The A's have 7 more game left against Houston than the Angels do. So if you subtract all of these common games, the teams have either 25 or 26 remaining games that are "unequal."
The disparate remaining schedules look like this:
Oakland:
Astros, 7 games
Royals, 7 games
White Sox, 4 games
Braves, 3 games
Mets, 2 games
Rangers, 1 game
Phillies, 1 game
Angels:
Orioles, 6 games
Red Sox, 7 games
Tigers, 4 games
Dodgers, 4 games
Marlins, 3 games
Mariners, 1 games
Indians, 1 game
Pretty clearly, of the A's remaining 25 remaining "unique" games, only the 3 against Atlanta would be regarded as tougher than average. Indeed, only Atlanta has a good shot at making the playoffs.
Conversely, the Angels get the 1st place Orioles 6 times, the 1st place Tigers 4 times and the 1st place Dodgers 4 times. And none of the non-1st place teams is a pushover. Yes, the Red Sox kind of suck on paper, but they have been playing better recently. If you get Lester, Lackey and Peavy, that's hardly a cakewalk of a series, and Buchholz and De La Rosa can be tough too.
Obviously, the head to head and common games count even more, since that's more than half of what each team has left. But when they aren't playing one another or common opponents, the odds are good that the A's are playing a team close to last place while the Angels are likely playing a team in 1st.