The site says it has sold 190,000 units. Tecmo has said that they have shipped 500,000 worldwide. You'd have to be out of your mind to think that their is really 310,000 units on shelves.
Link: http://kotaku.com/gaming/ship-shape/500000-ninja-gaiden-sigmas-out-290452.php
Unless, their is a huge number of copies pushed in Others ...... I know all the admins try really hard on this site, but there are a bunch of games missing EU and Others sales.