I have switched my FitBit account to Google and can no longer export data using custom dates but I can extract data from Google Takeout zip file where the data is stored in JSON format.
However the step count in my Google takeout jsons differs from the step count in my Fitbit app even after correcting for time zone. Note that I am referring to Fitbit, not Google's Fit app.
Weight data and floors (altitude in the JSON export) appear to be the same as recorded in the Fitbit app, unlike the step count which is almost always incorrect, often markedly.
It can't be a syncing delay as the errors are evident even when looking at the two data sets from many months prior.
Why does the exported step count differ from the app's recorded step count?