Interesting. Is this because of officially shorter working hours per week, because of longer hours on the other weekdays, or just some kind of an unofficial practice? How common is this?

As another Nordic, I've seen people gladly leave the office around 15:00 or 16:00 (or earlier for some people if they like to start early) on a Friday, but it's hard to imagine people who are on the clock regularly leaving at 13:00 unless they are real early birds.