It all started with IOS version 4.3 which was released to developers. Details suggest that Apple would opt for a change of GPU (Graphical Processor Unit) with a SGX543 (a driver is listed in the IOS 4.3) instead of PowerVR SGX535 introduced the iPhone 3G and present in the iPhone and iPad 4. And this new more powerful GPU would aim to support the resolution of the iPad 2. The iPhone 3G had a resolution of 480 by 320 pixels and iPhone 4 saw the resolution doubling, then to 960 by 640 pixels with density 326ppp (pixels per inch) of its Retina Display. But the iPad would follow the same (r) evolution. If it has a resolution of 1024 by 768 pixels at the moment, it could double the iPad 2 and thus pass to 2048 by 1536 pixels (ie number of pixels multiplied by 4 pixels per inch and doubled).
This is version 1.1 of the application iBooks (as well as version 1.2) that was inadvertently left a fourth icon to show its application with the indication 'iPadx2 "(the others being that of the iPhone, that of iPhone 4 and that of the iPad). And the resolution of the icon "iPadx2" iBooks to the application is twice as large as that for the iPad (ie twice as many pixels per inch).