But here's the explanation:
We also explained the “bit of misleading math” the campaign used: It divided an estimate for national health care savings by the U.S. population and, as an Obama adviser told us, “consider[ed] a 4 person family.” The president said he would “lower premiums,” but the Obama camp was counting on trickle-down savings that would affect families in the form of reduced taxes, higher wages or lower premiums. As we said at the time: “Obama claims families will save $2,500 under his plan, but they won’t see at least some of those savings directly in the form of lower premiums. And they may not see them indirectly either.”
Obama was talking about a reduction in health care spending compared with what would have happened without his health plan — not a straight reduction, though he didn’t explain that in his comments.
Source: http://www.factcheck.org/2013/11/fighting-premium-spin-with-more-spin/
Basically, Obama is counting on a trickle-down economy and some fuzzy math to keep his promise.