There is a new Android Lollipop build available for the Nexus 6. Android 5.1 only reached that device a week ago and an update is already available. The new build, LMY47E, includes updated radio firmware and a few other changes that most likely won't be noticed by the average user.
That radio firmware is responsible for Bluetooth, WiFi, and cellular controls and while all of those things are important, the update seems insignificant.
If you would like to download the factory image as fast as possible, you can get it HERE.