The reason Cost was so accurate was that he actually took the time to meticulously examine population changes within counties in addition to simple voter registration numbers. He found that while the Dems indeed added significant numbers of new voters, they already lost similar numbers in the same districts due to population shifts. He didn't say, but I'm guessing that it was just one set of Dems moving out and a new set moving in. It was that level of analysis that allowed him to show that the GOP had, in fact, equaled or even out-registered the Dems in these states.