The battery gauge is based on a direct measurement of the battery voltage. As the battery voltage fluctuates depending on its load, the gauge indication will also fluctuate.
For example, when you are riding uphill or accelerating, a lot of electricity is being drained from the battery and the battery voltage will drop. Thus, you may see a drop in battery capacity reading on your screen. When the ebike is stopped, you will see an increase in the battery gauge reading.
Do rest assured that this is completely normal for all voltage-driven battery gauges.