Why do brownies made from commercial mixes form a flaky top? When I use brownie mix products such as the standard Betty Crocker or Duncan Hines - the ones where you just mix up the ingredients, and then put it in the tray and bake it - I notice that the top just magically becomes flaky and delicious.

What causes that? The top is composed of the same mix as the bottom and sides; I realize it's exposed to the air, but how does that make the difference?
If I could offer a possibility..... When you bake a loaf of bread, the surface, exposed part of the loaf has a different crust than the interior (even when baked in a pan). This is due to the carmelization of the sugars on the surface. I suspect that this is the case here as well.

I get the same effect baking a mix or a scratch recipe. I've always assumed it is due to the carmelization of the surface sugars coupled with a bit of dehydration due to direct heat exposure....