Why IPhone6 Plus's logical resolution is 2208x1242, and the actual screen resolution is 1920x1080

Because all but the iphone 6+, the DPI of all other iphones is the same, all 326, with @2x footage.
But 6+ the actual DPI is 401, theoretically Apple should use 401/326 * @[email protected] material, but this wonderful proportion of the developer is very difficult to cut, so Apple for the convenience of developers using @3x material, Then zoom to @2.46x, which is actually scaled to 2.46/3=83%, and Apple chooses a close-to-scale 87%.
In this case, the ratio of physical resolution to virtual ratio is 87%, which is 1920/0.87=2208,1080/0.87=1242.
The advantage is that developers are more convenient, such as the preparation of material, the size can be directly adjusted to 3x. Source: http://www.zhihu.com/question/25288571

Why IPhone6 Plus's logical resolution is 2208x1242, and the actual screen resolution is 1920x1080