These are Calendar Quarters being represented in this chart.
In Quarter 4 2012, Apple had just introduced the 5" diagonal iPhone, so iOS users found a way to buy a larger iPhone they liked. In 2014, the 5.5" iPhone 6 plus appeared and the iPad was cannibalized by the sale of the larger phablet iPhones. A lot of people so no reason to buy two devices. Some of us with failing eyesight (at least until we got our lenses replaced and got back our 20/20 vision) still bought the larger screen iPads.