I wondered the same thing when the iPad 2 was labeled as a "marginal" upgrade. Any time you more than double the performance of something, that's hardly a marginal upgrade. No one would consider doubling the horsepower in their car a marginal upgrade. I'm not sure why we see it applied to iOS devices.

Maybe it's because the performance numbers are abstracted away so much more for mobile devices. Most mobile phone buyers probably view their phone as a black box. It does what it's supposed to do (hopefully) and that's about it. They don't know or care about the internals, so the outside gets so much more attention. If the outside isn't updated then it seems like it's a marginal update.