How do I know Runn's incline is accurate?

How does Runn measure incline?

Runn uses fairly high precision sensors similar to what you find in today's smartphones to measure the angle of the treadmill. 

Why is my Runn incline different from the incline on my treadmill?

Because the incline sensors are fairly high precision, it will measure slight changes in incline. For example when you get on and off the treadmill at the same given incline, the incline reported by Runn can be different based on the flexibility of your treadmill.