0.1 km is just 2% off 5 km which is within the accuracy of the GPS in the watch. Up to 5% incorrect distance seems to be within acceptable accuracy from what I can read here in the forum from Garmin officials, but you shouldn't see that much inaccuracy all the time. Under good circumstances you can get close to 0%.
GPS distance also depends on the number and sharpness of turns on the route you are running. If there are several sharp turns on your 5km route, the GPS distance may be shorter than the straight 5km route. Tall buildings or dense forest also affect the accuracy of the position and possible errors in measuring the distance.
