I was taking to a friend at work about it the other day and he mentioned something about stress test units running at a lower rez.
This story probably is a little more recent and seems to make more sense of the situation
Xbox One graphics currently output in 900p, worse than PS4′s 1080p (Updated) | ExtremeTech
The 2 most interesting points (2nd one backs up what the workmate said)
Update, 4:38PM ET: We’ve spoken with a representative from Microsoft, who provided the following statement: “Xbox One fully supports native 1080p output at 60 fps. It is up to individual developers to determine what resolution best fits their design goals. At launch, Xbox One will present all titles at 1080p—whether natively or upscaled.” It’s worth keeping in mind that just because a box can output at 1080p doesn’t mean it will.
An Xbox One developer took to Reddit and responded to the 900p claims. The developer (who has been confirmed as such), stated that the 900p claims were true, but that the resolution was output from consoles specifically made for stress testing. This means that the performance of those consoles should not be indicative of the final, retail unit’s performance.