There's a big of dual carriage way that I do on the outside lane every day.
Granted it only lasts about 300metres with slow moving traffic, so not exactly comparable to 2 miles
The reason I do it is that it's safer than trying to cross to the outside lane near the roundabout at the end of it, and the buses regularly use the other lane causing a variety of hazards.
Thinking about fast moving dual carriageways, I'm not too sure there's much difference in outcomes between being hit by a car from behind at 80mph in the outside lane or 50mph in the inside. Possibly the cyclist has reasoned that drivers in the inside lane are more likely to fall asleep, and despite moving slower still pose more risk. Not to sure I'd have the balls to put such reasoning into practice though!