We get a lot of people using our outside lane simply because they drive giant trucks that don't fit into the interior lanes. We are a farming community, so I'm sure that's not the case in most areas.
That's true at my FI as well. We're a small town with a lot of people driving really big trucks - many of them will use the last tube as well, since there's no barrier on the far side so they don't have to worry about trying to squeeze their vehicle into a very narrow space. That tube also sits a bit higher too, to accommodate taller vehicles. The big trucks are pretty much the only ones that don't get a side-eye from me for using the last tube when all the other lanes are open.