I would check the meter on some other shape than a wing and do this same flip to see if the readings are identical. And try it on a few different thicknesses. If the jaws do not have extactly matching V's then this much difference might well be the case. Do the tests on something very firmly held too so you know it's not shifting around.

Assuming it's the jaws that are causing the difference the actual reading will be the average of the difference. So your true wing angle is between -0.125° and -0.25° so call it around -0.18°. And I'd suggest that if you're not flying at some world championship level that this will be close enough.