Downloaded data, from Google Takeout, is not what the phone Fitbit app shows for same day and time.
As example, Heart Rate data from Google Takeout for one day is different than Fitbit App for the same day and time, including but not limited to minimum and max values for that day.
Additionally, the Fitbit watch is off my wrist once or twice a day, when this is so, the Fitbit App displays a lack of data for that time, while the downloaded data has data for the same time.
Finally, I left the watch off my wrist for a 24-hour period, and the app data is blank, the downloaded data contains data for that same 24-hour period.
The data downloaded from Google Takeout has Timestamp data that is using UTC time, and this is corrected to local time in the spreadsheet created to display the downloaded data.
