I see this argument a lot but I don't see any evidence whatsoever to support it. DISH's ARPU increased more than DIRECTV's did this past quarter. This suggests your claim is suspicious if not bogus.
The subscriber count alone supports this theory as simply put directv has way more subs than dish. Also, the reason why dish's ARPU is bigger than directv is because Dish nickels and dimes you more than what directv does especally when your looking at 17 dollar DVR fees. Given that dish has less of a pot than directv does, and charges more per user than what directv does the ARPU is going to be bigger for Dish than directv. The evidence is out there to support the theory but you cant look at dish's ARPU because they charge for different things more expensively than Directv with less of a sub base.