Generally, manually logging an activity is meant just for times when you were not wearing the tracker, though I realize people often want to use that for when an activity was not automatically recognized. When you manually log the activity, it takes a guess from the parameters you input for the logging. If you delete the activity you logged, you would get back the data that seems to be lost.
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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