Chris Ereneta:

Apple’s marketing screenshots have typically existed in a space of idealized fantasy, much like the gleaming, immaculately lit aluminum, steel, and glass of the hardware product images, each one resting on an invisible surface whose reflectance seemingly varies in relationship to the available negative space.Yet here I was seeing a deliberate choice to suggest a decrease in pixel resolution. On the FaceTime screens only.

This was clearly done to keep themselves out of trouble for advertising the quality of the screen. Had the antenna issue not popped up my guess would be that people would be bitching about the video quality of FaceTime (which is not perfect – but it is superb in actual usage).