Like any tracker worn on wrist, it can only infer steps from regular rhythmic swinging of your arm as in walking motion, so it cannot be precise. On a treadmill, if you are holding hand rail with fitbit wrist, it will not be moving so steps cannot be detected. Same issue pushing baby carriage or shopping cart. Likely the same issue on stairmaster if you arms are not swinging with your leg movements.
On the other hand, sometimes non-walking movements can be mistakenly taken as steps, such as strumming a guitar, playing drums, knitting. Fitbit has some filters to try to catch these but obviously is not precise.
At least calorie burn is based on heart rate, not steps, so is not affected by inaccuracies on step count.
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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