Charge 4 is not tracking my GPS distance accurately

[frbock]

On a 20 mile ride 3 weeks ago, the bike odometer and the Charge 4/ Google Pixel were within 500 ft of each other. A miracle!
We did a ride today, and the odometer registered 22.1 miles. The fitbit logged 15.96, which is a more common discrepancy.
Neither ride was in heavily wooded areas.
So, how do I get them to be on the same page on a reliable basis.

Moderator Edit: Clarified subject

[MarreFitbit]

Hi there, @frbock. Thanks for stopping by in the Fitbit Community Forums and sharing the details about the issue with your Charge 4. I understand where your concern is coming from. 

Fitbit devices use the formula below to calculate distance. Your stride length is determined by your height and sex. Steps x Stride Length = Distance Traveled

Please note when you track an activity with GPS, your device calculates your distance using GPS data rather than steps. If you begin moving before you get a GPS signal, your device calculates distance using your steps until GPS connects.

To update your stride length, track a run with GPS or measure your stride length manually. For more information, see Can I change my stride length in my Fitbit account?

For more information about GPS, see How do I use GPS on my Fitbit device?

[frbock]

Not as useful as I would have liked.
I'm riding a bicycle that has a wheel rotation sensor, and a constant distance per revolution.
I'm much more interested in why on most rides my fitbit is missing about 25% to 30% of the miles ridden. Yet on one ride, it was within 500 fit of the bike computer on a 22 mile ride. That ride was within 0.43% between the 2. I've done the same ride at other times, and have had the normal 25% error.