What were you doing with your hands while you were sitting? Your tracker is on your wrist so it can't really track your steps precisely; it can only infer steps from the movements of your arms and wrists, the normal swinging of your arms as when walking. If you were knitting, or petting a dog or stroking a cat while sitting, those movements might have been interpreted as steps.
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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