Quoted from GotAQuestion:Not only are phone apps not that accurate, many phones have a camera that sticks off the front now. I can't even lay my phone flush on a flat surface because the camera raises the top-left corner a bit over 1mm.
My phone is that way. I still use the app, but place a small piece of my kids wooden train track under it. Then I just make sure the ball doesn't move to much to one side. Maybe not perfect, but it works well.