How can a solo home run not be an earned run?

So the other day I was watching the Brewers-Red Sox game. In the first inning, Jon Lester gave up back-to-back solo shots to Rickie Weeks and Corey Hart. After the homers, I looked at MLB's box score for Lester, and it said he had given 2 runs, but only 1 earned. Assuming this was a mistake, I waited until the end of the game to look again, but it said he had given up 4 runs, just 3 earned. So, if anyone knows, can you tell me why one of those solo homers wasn't earned?