A quick experiment with T.G. as the subject:

Total posts: 6389 [3.45% of total / 1.24 posts per day]

But "Search found 5851 matches" when "Search for Author" was set to T.G.

How come there is such a big difference between the two numbers?

First, some of the posts may have been deleted, but the counter does not get decreased when that happens.
In addition to that, the search results are filtered, since there are some sub-forums here that are not publicly available - some available to moderators only, for example.

Really?

If so, how come I could pull off the 6699 trick?

Ah, I see. This must be "a general quirk" of the board.