Apple also uses a similar strategy with the iPhone. The iPhone 4 and 5 both made major form changes (retina display and screen shape) whereas the 4S and 5S were focussed on refinement (faster CPU, 64-bit CPU respectively). It has also been noted (possibly on the Accidental Tech Podcast, or somewhere similar) that a 'tick' year for the phone tends to be a 'tock' year for iOS and vice versa; iOS 5 and iOS 7 (released with the 4S and 5S respectively) were bigger changes than iOS 4 and 6.