That's because the numbering only gets bumped for sufficiently "major" upgrades, typically marked by a redesign. There was the original iPhone, then there was t

That's because the numbering only gets bumped for sufficiently "major" upgrades, typically marked by a redesign. There was the original iPhone, then there was the 3G (which introduced 3G and a new design) and its minor upgrade the 3Gs, then the 4 (which introduced the retina display and a new design) and its upgrade the 4S, then the 5 (which introduced the new aspect ratio).