I wondered that but it's on 8.1 already which is the most recent version. Besides, it offered me about five years worth of backups, going back to watchOS4. It's done now, it wasn't a massive effort to set up, it's just remembering how some of the custom faces I had set up were configured.I think it’s due to software versions.
/edit - Yes, confirmed. When I go to settings/iPhone storage/watch on my phone, it lists the backups. It's made one now but the most recent before that was a year old. They go back to April 2016!