Obama's evolution on marriage (his public one, at least) and Axelrod's admission now that Obama always felt that way is quite convenient and has served him well. But there has long been evidence that he held this stance prior to 2008, even as some of that evidence was disavowed by the White House.
In 1996, as an Illinois state Senate candidate, Obama indicated on a questionnaire that he supported same-sex marriage. In 2011, however, White House communications director Dan Pfeiffer said the questionnaire was filled out by someone else and that Obama "has never favored same-sex marriage." Obama also frequently said attitudes on the issue, including his, were "evolving" -- something many took as code for what was to come.
Indeed, as Axelrod writes, Obama told him at the time that he was "just not very good at bull*****ing." And he wasn't. Plenty of people believed at the time that he was actually pretty clearly in favor of gay marriage and just didn't feel comfortable saying it. Yet the White House apparently decided to keep up the charade for years afterward.
And Axelrod's confession, in and of itself, is pretty convenient. Yes, he's making pretty clear that Obama lied (which will hurt in the near term), but he's also making pretty clear that Obama was progressive on gay marriage much longer than he indicated publicly. He's casting Obama as a victim of the political winds of the day and of his advisers -- Axelrod included -- in a way.
He isn't the only president to change his stance on marriage as the country has, of course. In 2013, Bill Clinton tweeted his support for some U.S. Supreme Court decisions, including its decision that the Defense of Marriage Act, which he signed into law himself in 1996 and was declared unconstitutional.
The change in Americans' opinion on same-sex marriage unfolded in less than a decade, forcing many politicians to adjust their views. It goes to show that with enough polling data trending in one way, our elected officials can be convinced to "evolve" -- or perhaps more aptly, finally say what they've really been thinking all along.