"...can't work out why people get through so little in an evening."

It depends on the amount of chat between the songs.

I have noticed that in neither folk club I go to regularly do we seem to get round more than twice regardless of how many folk are there. On a week when the turnout is small, there tends to be a lot more chat between the songs so we don't get through so many.

It also depends on the MC. Some like to move things along and some are quite happy to let the discussion flow a little.