Nexus 7 gets update to Android 4.4.4 one day after Google posts the factory image for the new build
The factory image is posted by Google as a way to help those who like to customize their devices. Let's say you have accidentally bricked your tablet, or don't want to miss an update that will bypass those who rooted their device. The factory image will allow you to update your device so it appears as though it just came out of the factory.
And that sets up an interesting situation. Those who don't want to wait for the Android 4.4.4 update to reach them, can decide to use the factory image and install the new Android build now. Or, you can play it safe and wait for the update to hit your tablet.