When a game is released on virtually every available system, it's not a big surprise to see said game top a month's NPD sales figures.
Watch Dogs, which is available for PS4, Xbox One, Xbox 360, PS3, and PC, is your top selling video game for the month of May 2014. The month's runner up, though, may come as a surprise: it's Mario Kart 8, released solely on the Wii U and the second to last day of the month (May 30th).
MLB 14: The Show, Wolfenstein: The New Order, and Minecraft (it just keeps on moving) round out your top five.
As a reminder, the NPD sales figures only include data from retail sales. They do not include any digital sales.