This is not unusual for any gps track. Your bike computer is measuring distance by the circumference of your wheel, while gps is laying down points at some determined rate (every 1, 10, 30 seconds) depending on the setting of your unit. I have mine set for 1 sec. intervals and it’s not unusual for me to lose 10% in distance. Anything that effects accuracy can round off a lot of switch backs. Convert your track from .gpx to .kml and plot on Google Earth to see the errors.