So their tablet will be at a higher API level than pretty much every Android device out there. You still can't use the latest OS features as a dev unless it's OK for the Nexus 7 to have exclusive features.

Or maybe Google has another go at direct handset sales, or they withhold updates to maintain parity across their flagship products.

At the moment being a "Google device" doesn't actually guarantee you anything.

Edit: I didn't think that through. The OS version is already out of sync! I guess that is what they will do.