Whatever the reason, it's been an amazing event to witness. It won't last though, 65nm will dry up and 285 pricing will settle in somewhere around $350 I'm guessing. Maybe not at first, but if 4870 x2 pricing drops more, 295 pricing will have to drop, and 285 has to find it's place somewhere.
You got 4870 1GB just under it, and 4870 X2 just over it. When you think about it, the 280 (and now 285) has been in a very sensitive spot. No exact competition, but in it's case that might be a bad thing. Someone can spend $249 and get at least 80% of it's performance (4870 1GB), or $400 after rebate and get more in most situations (4870 X2).
Actually when I think about it, the sweet spot ideally is $300-$339 for the 285..