The kilometres differ because they are measured in different ways.
The RX chip uses GPS to determine the distance. To do this, the chip retrieves a GPS signal at regular intervals during the journey.
The resulting data points are combined at the end of the journey to give the total distance.
As the signal intervals are different for different devices, it is possible that additional end devices (e.g. Garmin bike computer) may output a different value.
The data in your on-board computer (Kiox, Nyon etc...) is calculated on the basis of a sensor on the wheel and stored in the system. The circumference of the wheel was previously stored in the system for this purpose. As this is a physical data source, this value is always the most accurate.