Retina is a term that refers to the point that a normal human eye can no longer see individual pixels at a normal distance from the screen. So for an iPhone, that distance is ~10 inches, and that is where the 300ppi number comes from. For an iPad, since it's held at a farther distance most of the time (~15 inches), doesn't have to have as high of a density to still be considered Retina. Hence why 264ppi is still enough to be considered Retina.
Steve Jobs explained it in the iPhone 4 keynote, and Phil Schiller backed it up in the new iPad's keynote.
EDIT: Damn, I need to learn how to type faster... lol