A common reason for small discrepancies in average speed between Fliiiight and other speed sensors is Fliiiight measures wheel speed at the wheel rim, whereas most Speed/Cadence sensors measure it at the tire.
Fliiiight also samples speed far more frequently than a standard sensor, which only updates once per wheel revolution, leading to small discrepancies in instantaneous speed between the two sensors.
Why the different approach?
Measuring speed at the wheel rim
Speed and cadence sensors measure speed at the tire because they are meant to measure your speed outdoors.
Fliiiight trainers measure speed at the rim because it's the rim passing through the magnetic field that creates the resistance, so this is most relevant for the power calculation.
Note that wheel speed in indoor training does not correlate directly with your speed on the bike outdoors, and is only relevant as a part of this power calculation.
It's also simpler to learn your wheel rim circumference than calculate how your rim, tire and tire pressure affect the outside circumference of your wheel. Make sure to update this value in the 4iiii app if you have a wheel size other than 700c / 29".
Sampling speed more frequently
In order to create virtual inertia, Fliiiight needs to sample speed far more frequently than once per wheel revolution- it needs to detect and predict speed variation within each wheel rotation. To achieve this, Fliiiight detects every spoke, using your speed index to determine how many spokes are in one wheel rotation.
For the sake of accuracy, we don't recommend pairing a separate speed and cadence sensor as the source of your speed reading in apps.