It appears that supply for Xbox hardware has outstripped demand. Speaking during Microsoft's investor call — following the company's Q3 earnings report — company CFO Amy Hood told investors that there will be a "channel inventory drawdown for Xbox consoles."
As Gamasutra points out, reducing the amount of available inventory implies that Microsoft has produced more Xbox consoles than it is currently able to sell. Hood added that the company "expects to work through some inventory" in the fourth quarter.
Microsoft revealed during the call that it had sold in (read: shipped, not sold to consumers) two million Xbox consoles in the quarter ending March 31. Of that, 1.2 million were Xbox Ones.
Last week, Microsoft revealed that it has shipped over five million units since its launch in November. 311,000 Xbox One units were sold in the United States during the month of March. Comparatively, PS4 worldwide sales (to actual consumers) have topped 7 million units.