Historically in this part of the world, pigeon shooting was for the rich and famous, and I bet a few would do it with 20 gauges just because they could.
Not all pigeon shooting was competitive. The pigeon shooting club was a place for a business lunch or meet some friends by the end of the day, and have a few rounds at pigeons while at it. This kind of informal shooting, plus the wife or daughter being able to shoot a few, may explain some 20's pigeon guns. I saw a Belgium one for sale some time ago

HM