I'm not sure I believe that yet, but there is more to the demands of an image than FPS and resolution. You have to take image quality into consideration too. It is possible...possible...that as games try to push the image quality of the new gen systems, that both FPS and resolution could suffer on the XB1.
Even though Shadow of Mordor is cross gen, it's development and feature list for last gen and new gen are different. Unlike say Destiny, they are not trying to push the exact same game to all systems. SoM is open world and can show a lot of characters on the screen at once. That can be quire demanding. It's possible that the developers decided that it would be better to keep their image effects and complexity at the expense of FPS and resolution on the XB1.