Oh, we figured this out, maybe I should update the page.
It turns out at installation time the installer set all the fans on "high", and I left them that way stupidly thinking: "Well, I like circulation, and the power meters show the fans aren't actually using much power, so I'll just leave it like that."
Changing them all to "auto" fixed the problem.
It turns out that while the fans themselves weren't using a ton of power, their activity caused the heat pumps feeding them to work harder. In retrospect it seems not that surprising and I feel pretty dumb about it.
(And yes there are a lot of windows but they are not south-facing and they are all tinted in a way that blocks most of the sun's energy.)