Some people have a health/activity program at work that provides an incentive like points to do a certain number of "activities" every day or week. Points can be exchanged for stuff or, in some cases, gift cards. So they're gaming the system with the walk to the mailbox.
It's also possible that Strava is getting a secondary feed from their activity tracking app/device that they set up in case they do any real workouts and they don't realize what's being replicated.
Or not.