Of course the units won't be divided evenly per store, but I don't think it's unreasonable to assume that, if StoreX accounts for X% of a company sales, then StoreX should get X% of that company's product.
Also, Wal*Mart accounts for about 35% of the total Video Game Market. I wonder what percentage of Nintendo's US sales come from Wal*Mart. Considering how unfriendly toward Nintendo that the second largest seller in the US Market is (EBGames/GameStop with, I believe, 11%), and considering the type of shopper that would purchase stuff at Wal*Mart vs. other stores, it seems to me that Wal*Mart could easily account for more than 35% of Nintendo's US Sales.