Good for you for checking this out.
I’ve seen this on treadmills, my own Nordic Track incline trainer is wildly inaccurate in terms of speed and incline. I have calibrated it using a mark on the belt and it is off by around 1min/mile!! I have checked the incline with a inclinometer and it’s way off too.
Semple has the same problem with his Nordic Track incline trainer. I think the best we can hope for is consistency: The grade may not be accurate compared to the reading on the machine but at least the grade does not change. As for speed, I have not tested how it changes over the short term (during one use) but The variation in speed must be relatively small as it is insensible to me when I’m on the t-mill.