First 4 are 18" excluding the youth models, then it gets into the sporter models with the longer barrels. I'm not a hunter so I often oversee these models. Surely you wouldn't use the KSG for hunting anyways, so naturally I figured we were talking about tactical application shotguns. Anyways, maybe its a stretch saying "most" 870's, but it seems to be a rather popular barrel length. Mainly because it's the shortest you can have by law, without paying the 5$ tax to have a short barreled shotgun.
My main point is, 18.5" is not a short barrel, for a shotgun with tactical applications in mind. 14.5", that's a shorty.