GPS can be very unstable, especially in city when signal is blocked and reflected by trees and buildinds. I do map with OSM2GO and I know how far location detected by GPS can be from me. This error changes over time so GPS speed is a summ of your speed and and speed of changes of error. So it is several kmph higer, than your real speed. In a car this does not mean much, on a bike this could be half of your real speed.
GPS can be very unstable,