Looks like a lot of people are asking the same question. Lego City and MH3U were released towards end of month and both had shipping problems.

http://www.nintendolife.com/news/2013/04/talking_point_the_terrible_timing_of_monster_hunter_3_ultimates_stock_issues

And the trend here at VGChartz lately doesn't lend credence to it being overtracked. If you've been watching weekly sales and Feb adjustments you'll know what I mean -- VGChartz had everything selling at a certain point in Feb, then had to adjust up massively after NPD numbers were revealed, but immediately started tracking everything right back down where they were in Feb again. No question to me, Wii U was undertracked in March just like everything else. It hardly matters what Pachter says, only three days before we know.

I find it hard to believe sales will be down so much with the releases there were, on top of it being a 5 week month.