It is common for there to be a difference between the speed indicated by your vehicle and the speed displayed by Radarbot.
This difference occurs with all navigation applications. By law, vehicles cannot display a speed lower than the actual speed. This is important for safety reasons and for the correct functioning of speed cameras, as well as to comply with road speed limits.
GPS-based applications calculate speed by measuring changes in position over a specific period of time. The speed recorded by these types of applications may be more accurate, although it is not completely infallible.
Remember that you can enable high-accuracy GPS to obtain a more precise measurement.