I'm sorry for not seeing this until now.
I own a Landice mill. I have the same issue. I've never measured how far out of level it is, but at 0% grade there is NO DOUBT an incline.
I wonder if the manufacturers do this to combat the lack of air and other friction that a treadmill doesn't provide.
I know me running on a flat road is actually LESS effort than running on my mill set at 0 degree incline.