If you are rated higher than someone else, the expectation is that you will win a game against them. Therefore, you will gain fewer points for winning that you would forfeit for losing.
Interestingly enough, if A and B have the same rating, let's say 1500, then if A wins a game between them, A's rating becomes 1516 and B's becomes 1484. Now A is "expected" to win any game between them (win expectancy = 0.55), so if he loses 17 points change hands: A now has a rating of 1499 while B is rataed 1501.
The practical implication of this is that, if you have a choice and if ratings are what you care about more than sportsmanship, you should take your losses before your wins.