Airfoil- concave top and bottom
It's because the "rules" people are traditionally taught about creation of lift, basically that a wing is a semi-venturi that is sucked up by venturi effect as it moves, is a gross oversimplification that overlooks most of what is going on, which is the wing motivating a large package of air to move down and creating a Newtonian action/reaction, of which the venturi effect is a part.

Anything that is more or less flat-ish, moved through the air at some angle, will make some amount of lift if it's able to induce some of the air to move from up here to down there as it passes.  The front door of your house, your hand stuck out the window of a car, or a wing with a weird reverse curve on top.  Enough to make an RC airplane fly at least.  Probably wouldn't work so well with a full size one.