So Damien and Marc from Hardware.fr ran a few tests on the HD 5830, and I think they figured out the problem:
http://www.hardware.fr/articles/783-3/preview-radeon-hd-5830.html
Now it makes sense!
That's a good catch - I'm surprised that more review sites did not hone in on the fact that this card essentially performs like a 128-bit card for AA situations.