Might be worth looking at the maps for both records closely. It's likely that one of them is a “zigzag” path (as a result of poor gps reception e.g. in a deep forest) which would appear longer; and/or the other one “cutting corners”, taking shortcuts where the actual path is longer. Could this explain the difference?

...also, you can try the “correct distance” feature in Strava (open the activity via the web interface - as it's not in the mobile app - in the left menu, click the three dots, and then Correct Distance), that can help if gps had troubles and resulted in some wildly inaccurate distance. Strava recalculates the distance based on the map data and as a result, you might get the same or very similar distance to the one in OS. (If you're not happy with the result you can revert that change afterwards.)