I think this echoes what DuffVader was saying. When a driver is closed at impact -- compared to its natural state -- it effectively delofts the club. My guess is that most pros have closed vs. square face at impact --compared to how the club sits a address. So, in order to have a square impact position is to start with a driver that is naturally open at address.
Open drivers do not have an effectively lower loft. What they do is allow a square impact position in a swing the closes the face at impact to a position more closed than at address. They facilitate but do not cause delofting the club at impact while allowing the face to be delivered squarely to the ball.
I hope my explanation made sense.
Josh
Edit: While I was replying demolition man gave an excellent explanation.