I got a Fenix 5 two days ago and it is acting absurd in recording steps. I will walk 50 steps and it'll count 150. The discrepancy is extremely high. If ill scratch my head, it'll count 10 steps.
It is perfectly normal. No money will make the fact that some non-walking arm movements look like walking go away - all trackers suffer from this with some edge cases being more pronounced than the others. The value of step count isn't in the absolute number but in the trend.
I am tracking steps with my fenix 5 and concurrently with my iPhone internal step tracker and the values are always very well correlated but of course never exactly the same.
Well my F5 counts lots of step while in a car, i go as copilot not moving mi arm and can see the steps counting up crazy in a no homogeneous road (but nothing wild).