This is among the most frequently asked questions since iOS 11 made it possible for RunGap to import full workouts recorded by an Apple Watch including route maps.
As described in this article on the Strava Support Forum, Strava analyses all uploaded activity data and recalculates distance and "moving time" from the raw data streams embedded in the workout.
If Strava finds that the distance or "moving time" is incorrect, it will adjust the workout accordingly. This will of course affect the average pace and other values based on time and distance.
Especially workouts recorded with the Apple Watch "Workout" app tend to show a shorter distance on Strava. This is most often caused by the route cutting corners and bends as shown in the example below. For some workouts this can easily shorten the distance by 5% or more.
The same route as recorded using the Strava app (red) and the Workout app (green) with the same Apple Watch Series 3. In this example it is easy to see why the green route leads to a shorter distance.
Workouts from other sources such as Garmin devices tends to have more accurate distance values and are seldom modified when uploaded to Strava.
Unfortunately it is not possible to bypass the distance recalculation without fiddling with the data - but the "moving time" calculation and it's effect on pace can be disabled by changing the workout to a "Race" on Strava.
Apple Watch users may prevent RunGap from reading the route maps from Health by disabling RunGap's permission to read "Workout Routes" under "Sources" in the Health app. This will stop Strava from recalculating the distance by sacrificing the map.
Similarly RunKeeper will also recalculate distances from the GPS data when uploading new workouts and Apple Watch users may get a shorter distance here as well. Again this can not be bypassed without sacrificing the map.