Google has quietly posted the Android 5.1.1 Lollipop factory images for select Nexus 10 and Nexus 7 tablet variants. Since this is an incremental update, the new firmware mostly brings bunch of bug fixes and improvements.
Google has been actively working with smartphone and tablet markers for successful rollout of the Android Lollipop to existing devices. It is a thumb rule that the Nexus devices are the first to get any new updates.
The OTAs are usually meant for everyone. Factory images on the other hand, are targeted at Android developers and enthusiasts who want perform a clean install of the firmware.