Here is a post about the accuracy of the cell phone inclinometer app:

https://pinside.com/pinball/forum/topic/setting-playfield-angle-with-cellphone-app-how-accurate
Post 3006209

Alan

Some of that could be due to phone design/feature differences. Maybe some phones work better than others.

The real test of a level is whether it reads the same in two opposite directions.
Do any of your apps read the same (but negative) angle when spun around 180 degrees on any surface? It doesn't need to be a level surface for this, any constant angle surface will work.

If not then the apps are not calibrated to your phone. There should be a routine for this in each app.
If you can't calibrate then in a pinch you can manually calculate the average reading from the two 180 degree readings.

Using the Clinometer app on my Android Note 4 phone I will get two different readings initially on a surface (spun around 180 degrees). I use their calibrate feature to take two readings 180 degrees apart and the app corrects the difference and the result is both opposite readings are identical.

I do this each time I open the app for a new session.

Alan