Even though Xbox One managed to caught up and sold over 2 Million 'worldwide" in the first 18 days while the PS4 sold over 2.1 million PS4 units worldwide and become the "Largest Console Launch in History."
According to a NeoGAF user jaaz, it appears that GameStop is desperate to sell Xbox Ones to customers.
"I've seen the same or similar set-up at least two other GameStops. They had a few empty PS4 boxes stacked but nothing near as prominent. Inquiries on whether the PS4 was in stock were met with a negative. In the last GameStop, the employee actually said he had a PS4 but was using it as a Kinect stand for his Xbone because the PS4 has no games, especially sports games. I didn't even bother with him.
Take it for what's it worth, but there seems to be a clear push at GameStop to move Microsoft's next-gen device. It also appears from the homemade signs that GameStop is still trying to clear the air from the Xbone reveal fiasco earlier this year." – jaaz