Unfortunately, sometimes even an “official” race route is short, as some organizers don't bother with getting the distance properly measured and certified. (A local “half marathon” is notoriously known to be around 20k, for example.)

Anyway. You can try the “correct distance” feature - you have to open the activity from Strava website, it's not in the app. Strava will recalculate the distance based on the map data, and it might recognize the HM distance afterwards. 
 

In addition to what ​@Jana_S stated above, it could also be that your device had poor signal for some or all of the activity.  GPS devices are not exact despite what many people believe.  They are just connecting the dots between two points over and over and over again.  If you have a very twisty trail for example, it will often under-measure the distance because it will “cut the corner” at times.  Also, if you have auto-pause turned on, it may not start recording exactly when you resume moving after a stop, missing a short bit of distance each time.  A weak signal due to obstructions can give inaccurate location data which can skew the measured distance as well.  

Other than the “correct distance” option mentioned above, another option is to just add a manual entry for the event with the half marathon distance and your official race time.