Actually, tis the E3 season and all, but with the high likely hood a slim model is releasing at E3, and MS are clearing out stock with $299 500GB bundles and $319 1TB bundles, is it possible that this low May result is because there is actually little stock in the channel right now?
Dropping 35-40% from April to May is a bit extreme, and would be more than the 33% MoM drop from April 2014 to May 2014.
There's reports about people not seeing Xbox One available? I believe MS are clearing stock, but the console isn't selling out yet.