Doing a 360x640 will indeed create a rather awkward situation where iPhone 6 plus have less usable screen space than the iPhone 6 (which is 375x667). Though, my guess is that they want to keep the physical height of elements to be roughly the same across all iPhone family.
This way, all existing buttons that were designed with 44px tapable height in mind or any existing text could have more details in it while not appear larger or smaller than any other iPhones and will have more space to display content. I believe non-mini iPad is the only product in the iOS family that recommended 44px is physically larger than the rest.
This way, all existing buttons that were designed with 44px tapable height in mind or any existing text could have more details in it while not appear larger or smaller than any other iPhones and will have more space to display content. I believe non-mini iPad is the only product in the iOS family that recommended 44px is physically larger than the rest.