It's just screwed up like that. At least it means I don't have to download 1.6 gigs, but it's annoying not knowing how much I have to download (it said 900-1000 megs when I clicked install though).
Happens to me too the download rate in the bottom box is often 1.5x the top. The top value for kb/s is the correct one, it matches up with the Activity Monitor value.