It's reflection plus air flow. Sunlight is blocked from reaching and warming the roof, while warm air rising at the top of the roof will pull in cooler air from the bottom, leading to surface cooling (sort of an accidental half-assed solar chimney). So, it's a couple of different mechanisms at work.
It's definitely one of those makes-sense-in-retrospect kind of things. I'm just surprised it never came up as a point in favor of the systems when discussing with the various solar sales reps- we spoke to several different companies, and not one mentioned it.
