After using a couple W7000s I think they are the same as other projectors (I have used over the years). Virtually all of them drift during warm up... from JVC, Sony, BenQ, Optoma and others... My guess is the different elements in the lens assembly move slightly as they warms up causing the focus to drift. The last one I used the image moved slightly to the right as it warmed up too.
So if you focus with the projector warmed up you are setting a point where it looks best after the elements are warmed up. When you first turn it on the elements aren't in the same alignment which makes the image out of focus. So it's not that the projector has better focus after it warms up... rather it duplicates your prior setting (the elements are at the same state) when it warms up. If you focus right after turning it on it will get out of focus after warming up but will look wonderful the next time you turn it on... at least for a few minutes.