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Some retailers decided to offer the Google Nexus 7 (2013) ahead of its proposed July 30th to their customers. Because of this Google started accepting orders for the new Nexus 7 tablet as well, even though they don't plan to ship it until the previously proposed date. The new Nexus 7 comes with Android 4.3 Jelly Bean which is the newest version of the software. But it turns out some of the first customers who purchased the tablets from retailers are already receiving a new software updates.The first update occurs when the tablet is connected to the Wi-Fi for the first time, likely during the initial setup of the slate. Once you install the update the tablet reboots and lets you complete your Google account sign up process. Once updated the device software will carry the JWR66N build number.
The second update doesn't seem to be mandatory and you will receive a notification to download it to your device. Once it is installed, your software will have the JSS15J build number which likely brings some minor tweaks and bug fixes.
Staples has already sold out their 16GB and 32GB stocks of the tablet while Amazon has sold out their 32GB variants. You can still check for availability at your local Best Buy and Walmart stores if you dont want to wait for Google to ship it out to your through the Play Store.
source - Android Central