Because they actually came out to vote. Adults/registered voters/likely voters are an unstable number, but actual voters who do go to the polls - for sure - are a "sure thing." The odds of them voting again are pretty solid.
2008 tells them what % voted for who, so they use that as their guide. Those same people will vote again (although, personally, I think a lot of democrats will just stay home. They won't want 4 more years of this disaster, but they won't vote Republican either).
Thanks for the response. The reasoning sounds logical except there is plenty of evidence to suggest that 2008 was an aberration due to many factors which won’t be present in 2012 (historical election, Obama was new and exciting esp. to young voters and even older folks who had never voted before, plus he had an overwhelming money advantage, and so on). Pollsters should also take into account what happened in 2010 and the recent polls showing Republican enthusiasm is higher than Democrats and that a significant number of Obama voters have become Republicans.
I believe Rasmussen has it about right. I don’t remember exactly what his breakdown is but i’m thinking it’s relatively even between the parties or at most a 2-3% advantage for Democrats. Anything higher than 5% just isn’t realistic in my opinion.