"One of the new production processes is the use of a two-stack tandem structure, which has two emission layers, thus doubling brightness and quadrupling OLED display lifespan. All of Apple's iPhones use a single-stack structure, and Apple's rationale for requiring two-stack panels for iPads is that tablets tend to be used for longer periods."
My first thought was that Apple were going overboard. But obviously the statistics they collect from there devices indicates there is a method to there madness.