5870s official TDP is 188 and 480's is 250W. If you're seeing several places that show it below 188 you're reading 5870 reviews in which power consumptions are measured. So the fact that you're saying "I'm also seeing places where 480s are closer to 290" means that you are also reading GTX 480 reviews 4 days before NDA, and there you are seeing measured power consumptions of 290W. You are making perfect sense.

Plus, 2 times 188 makes 376. Still not 290.