calibrate level

Anyone know if it’s possible to calibrate the Level function in the Measure app?

I noticed that if I hold the phone upright and tilt it back and forth the horizontal line moves up and down the screen, looking like it’s telling me whether the back of the phone is vertical or not. Except it’s wrong. Here’s a screenshot of the phone held against a vertical wall. And yes I checked with a spirit level. See the green line is not where the marks are?