I posted about this too, I had 8MB where 1.2MB would be the absolute maximum attainable.
I too thought voodoo at first, but maybe data compression is a more likely cause than voodoo.
When downloading an update/patch, not all files have to be downloaded, only files that have changed. However, I think that when the downloader gets to a file it doesn't need to download, it skips it but counts it's size in the data it's downloaded. This is where the silly download speeds come from I think. So if the game was 1GB in total and the patch only needed to download 1MB, then the downloader would report that it had downloaded 1GB in the time it took to download the actual changes.
This is all theory of course