how about it's the freaking table you have it set on that is glued together strips of wood?
I can see it in the table that they aren't quite level.
But (as the OP has already said) if the problem was with the wooden surface then the phone would exhibit the same rocking motion both ways up. The phone is clearly developing a hump back.I think your table is bent, you might want to ask your furniture vendor for an exchange.
Seriously though, based on that quality of wood, I would expect a tiny bit of imperfection.