You mean the update that came out weeks ago (91) which was a tweak on top of the single Froyo release before it (85)?Google with the FRF91 update. Why do you think updates kept coming out so quickly. Google has a ear to the ground, they put the Nexus out there as a test of Android OS, and they want feedback to improve it. It's all about feedback and improvement.
The 50/70/72/whatever were all beta testing and were never sent to users.