It doesn't. I guess some people just want the numbers to be more overtracked than they really are, hence making the weekly numbers MUCH smaller. Here's the real math. In the Wii's 2nd Xmas, it sold 1.1M in the corresponding weeks we have left on VGC. Right now, VGC has the PS4 at 18.2M, so 300K left to reach 18.5M. It's obvious the PS4 didn't match the Wii's sales, so let's say it sold 900K-1M in those last weeks. This makes just as you said, 600K-700K overtracked.
Knew I wasn't crazy!