I think the speed shown and calculated by GPS phone is not 100% correct. If we had better receiver in GPS device we would have better accuracy. This can be explained in this way, when we are moving with very low speed then the observed error will be high but if we move with higher speed then observed error. Still we shall have same amount of positioning error but it will be distributed.
With Nokia map that you mentioned, they have the same problem. You can test it by yourself. Just start the map, and tell the map you are driving and observe that it will show your speed 3, 4, 5, 0 etc. Km/Hour (They are calculating the speed and depending on the error they show the result, it may be accurate may not be)