I think you are reading too much into what Zogby says. He could have been talking about independents, or difference between R and D, etc. I don't think you could do assume a simple set of numbers to get 49%. For example, you average 48, 49, 50. You can have a wildly different set of numbers like 60, 45, 45 to get 49%. So if 60 rolls off, then the tracking numbers change by a big amount.
But I agree with you that Zogby is full of shit. I think we should all throw out Zogby's #'s, and concentrate on reall polls like Gallup and Battleground #'s.
I second this motion. :) It is fun -- sort of (maybe I just need to get a life) -- to monitor these daily polls, but the truth is that when Gallup is away, the mice will play.