The Nexus 7, everyone's newest favorite tablet, has already had more than its share of problems shipping from the Play Store. Today, the new shenanigans are that the 16GB model is currently unavailable from the Play Store at all. The best Google has to offer is a "coming soon" button that offers to let customers sign up to be informed when the device comes back in stock.
The 16GB model, so far, has been particularly hard to find from Google, and particularly easy in retail locations. The best we can assume is that Google allotted more 16GB models to be shipped out for retail shipment, since the margins are higher on the 16GB model.