Hello @Tweedlemarina 

Thanks for posting about this.  

We are two separate platforms with different calculations so certain aspects of your activity analysis may be different. In general, this is because Strava is more strict about resting time so we remove less of it than Garmin Connect. In other words, we're a bit more likely to consider you moving in places where Garmin considers you stopped. This means that even though the elapsed time is the same, the moving time can be a bit different, which can cause differences in average speed. Strava typically reports a bit more moving time and so a bit slower average speed.

We have have an existing topic on this subject I would encourage you to check out.  You can view it here