I see a similar pattern for site:wikipedia.com - a really weak showing as far as I can tell.
This pattern of site: SERPs not going nearly far enough has been growing in recent months. Used to be if there were over 1,000 indexed pages in a directory, then site:example.com/directory would return 1,000 pages. Then it was almost 1,000 pages. Then it started falling lower. And now it's lower still.
I honestly think this limitation is an artifact of some new data processing method at Google, rather than an attempt to hide data. But you never know when it comes to publicly available information. After all, the public link: operator has never been complete.
If Google is intentionally hiding some of the data from public scrutiny, I would understand that. However, I would also then like a chance to see my own full data within Webmaster Tools. Seeing an accurate picture of URLs indexed (by directory when appropriate) would be a very helpful service.