Okay, well I think I've found a reason: network-shares-automount. I don't understand it, because the shares are not mounted in media/drive1 (they are mounted in media), but removing network-shares-automount restored the proper stats, and reinstating it recreated the reporting problem. Maybe this is a bug in the HD-FOX firmware?
In the process, I tried removing flatview to no effect, and also discovered I had foxlink installed (which I don't need and had forgotten all about - but explains a mystery share!).