Oh, I see why the mailing lists sometimes default to replying to the list and sometimes to the sender... if I'm in the To:/Cc: explicitly then when I hit reply it goes just to the sender. If I'm not, it goes to the list. Ah: because the ones explicitly to me are not even going through mailman (even though mailman is on the Cc:). I think there's a setting in each subscriber's mailman to also (or not) send a copy when you're already on the Cc:.
A bit frustrating as there's no good solution. If I tell mailman to send me a copy too, then I'll always get 2 copies. And I can't tell the origin mail server not to send me that other copy... Unless someone can think of something I haven't, looks like using reply-to-all or reply-to-sender explicitly each time, and never using just "reply", is the only way to ensure the MUA is doing what I want.
Or... I guess if everyone refrains from ever letting any individual person's email address get into the To:'s and Cc:'s that mailman sees... good luck with that!