The One is a clip-on, generally worn on waist or hip; you Alta HR is probably on your wrist. Clip-ons generally can better sense steps by feeling hip movement.
Something on wrist cannot directly tell what legs or hips are doing, but try to tell from sensing the swinging the arms as in normal walking. This can lead to some over or under step counting but generally over the course of a day evens out fairly well. It might overcount when doing any regular rhythmic wrist movement such as knitting, playing drums, etc, and undercounts when the wrist is held steady such as holding hand rails on treadmill.
Purely for step count accuracy, clip-ons might be more accurate, but wrist trackers provide more information (such as heart rate), features, and convenience.
Before posting, re-read to see if it would make sense to someone else not looking at your Fitbit or phone.
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