I can add something about Milan that could explain those data. Milan has some water channels going at least halfway into the city. Some bicycle roads follow the channels and they don't have many intersections. Some 10 miles each way are believable. People doing 10+10 miles each day on those roads could be the kind of people that also record their commutes with Strava. However people that have to commute on normal roads shared with cars probably can't be that fast.

I live inside the city and I never mount my bike computer when moving around in there. The normal cyclists that use a bicycle to go shopping probably don't do more than 12 km/h, twice as much as a fast pedestrian. Obviously they don't record their journeys.

So, still skewed data but with an explanation.