Have I been doing it wrong all these years?

The tow mode does not have much effect when towing on flat roads at crusing speeds. It makes the engine rev more when starting from a stop (which uses a bit more fuel), and it causes the transmission to downshift when you apply the brakes on a downhill grade. But you knew that, of course.

I wonder, were you were towing with a tailwind? I have noticed a huge difference between towing into a wind and towing with the wind behind. I would not be suprised at the 3 mph difference if there was a good tailwind.