The specifications claim accuracy of +/- 0.2 degrees. Your readings differ by 0.3 degrees, which is within specs (+0.15 degrees in one direction, -0.15 deg in another). Also the device might be slightly rotated off axis if you manually place it at playfield.
Probably it would be best to take readings in both directions and then average them for most accurate result. (your playfield would then be at 6.85 deg.)