The other side of this, is that market supply/demand conditions at the distribution point may mean that Premium is being sold from the Regular tank.
My wife used to work in the Shell trading-processes area and describes the following scenario - Premium is not selling as well as forecasted, and this information has yet to be relayed (and realized) back through the delivery stream. Meanwhile, Premium is still coming through the system at the erroneously high predicted usage.
Something has to give, that gas has to be put somewhere. Therefore Premium becomes Regular. This happens at, or prior to, the distribution rack, where the tanker driver loads. This is most likely invisible to the driver, and definitely invisible to the service station. So the "Regular" on the truck is sometimes actually Premium, therefore confusing the customer who sometimes wonders why his car doesn't run any better on Premium.
And of course, per my previous post, the Regular illicitly going into the Premium tank is sometimes actually Premium.