Then it seems clear that many of the URLs are being filtered before your domain's spidering is made available live on google.com. The most common reason for this kind of filtering seems to be link juice (PageRank) but if you do an analysis of your own case, you may find that some other characteristic is involved.
For over a month I see an extra 2M pages for our site: if I check from home compared to the office (not logged in). Parent company is also the country's biggest ISP, so a colleague using that ISP from home gets the same count as at work.
The only difference I can see is that my home ISP is not powered by the employer's ISP arm. Yes, I typed ISP, not IP. Weird.