In defense of a wider/more horizontal form factor, it sure helps when displaying more than one app at-a-time. Which coincidentally is one of the main selling points of Win8.
I know that sounds a bit snarky, but it sure sounds and feels better than fuzzy math.
It's also better for most video content; an iPad watching a 16:9 TV stream is only using 75% of the screen. Apple itself has moved to a widescreen format for the iPhone 5, and I wouldn't be surprised to see a future iPad do that too. I think there's lots of value to squarer aspect ratios for desktop/text usage (c.f. the Chromebook Pixel moving slightly in the opposite direction), but for a media device like a phone or tablet I'd expect them all to converge on the aspect used by content.
Obviously none of that has to do with the bad math in the MS ad. The iPad screen is, in fact, larger than the Asus one.
I know that sounds a bit snarky, but it sure sounds and feels better than fuzzy math.