The GPS is takes positions periodically. That means the GPS track is going to be straight-line segments between each position it records. Plus, there's some error (inaccuracies) in the GPS position data.

The wheel sensor, on the other hand is much more accurate for distance travelled. The GPS and the wheel sensor distance should be very close the more your actual riding path is a straight line (or made-up of relatively long straight line segments).

GPS will also be more accurate for higher rates of speed (much higher than you will typically be riding at).

GPS will often register a small speed when you are standing still. That's the error (position inaccuracies) telling the GPS that you are "moving".

A dedicated GPS unit might give better results than the one in the phone too.