Wow - I didn't realise the list membership was so small! (considering the size of Wikipedia) Also 266/901 is a rather hefty proportion on moderation I would have thought - almost 30% of the total membership!
I would imagine that the vast majority of them have never posted, and hence never been automatically taken off moderation.
That, and moderated people are more likely to have multiple email addresses signed up than non-moderated people. In terms of people, it is probably a significantly smaller proportion.
As for it being small, I'm not suprised. Only very active Wikipedians would be interested in the mailing list, and not all of them. In September 2006 (the last month I can find stats for - do we have stats anywhere from after the toolserver failed?), 4330 accounts made more than 100 edits. Accounting for growth, etc., I'd say about 10% of very active Wikipedians are on this mailing list. Seems about right.