The "Romney advantage" is something Gallup made up by adding the "conservative advantage" to the "Republican advantage."
Based on the assumption that ALL Republicans and conservatives will vote for Mitt Romney. 20% of self-styled conservatives, nationally, voted for President Obama in 2008. I'd be a somewhat socially conservative Democrat but it sure as hell doesn't follow I'd be voting for Mitt Romney
You make the false assumption of "all" or nothing.
If you look at the 2008 Gallup results for the two categories, you will find a very high correlation between actual vote and predicted results.