This makes some sense to me. The bulk of my Wikipedia contributions by word count happened when I was a rank newbie. Today I'd probably be too self-conscious to make them, too conscious that this article isn't encyclopedic enough or that article has what somebody thinks is the wrong tone.

Also, I suspect that the major impulse for starting to write wikipedia articles is, "oh, shit, is that ever wrong/incomplete!"

And once you've written a few articles to correct that flaw, the overlap wherein you have a clue and wikipedia doesn't shrinks accordingly.