At Google I/O 2012 event, when Android 4.1 Jelly Bean was unleashed its SDK was also released to developers but now again Google is pushing Android 4.1 Jelly Bean SDK which is also a developer version but its the final and embedded with fixes.
By releasing Android 4.1 Jelly Bean SDK, Google means to encourage developers to make their apps compatible to the upcoming Android OS update, whereas Nexus 7 is already reached in testers hands.
According to official blog:
“For many people, their first taste of Jelly Bean will be on the beautiful Nexus 7. While most applications will run just fine on Nexus 7, who wants their app to be just fine? Here are some tips for optimizing your application to make the most of this device.”