The finding that people with an "Overweight" BMI have a lower rate of all-cause mortality is consistent and has been called the "obesity paradox." A very interesting study came out this month that addressed it.[0] Basically, they did a prospective study of 55,000 (mostly elderly women) where they measured their body fat percentage directly using DEXA scans. It turns out that when you control for body fat percentage, it's actually better for all cause mortality to have the highest BMI category and the lowest body fat percentage. Of course, the measure "BMI after controlling for body fat percentage" is kind of unintuitive, since BMI is supposed to be a crappy proxy for body fat percentage. But basically, as long as your body fat percentage is low, being bigger is healthier than being smaller.

0: http://annals.org/article.aspx?articleid=2499472 (this paper used to be available, because I read the whole thing, but I guess it's paywalled now)