Why are cambered airfoils used for horizontal stabilizers in some designs?
It's done to get a little more lift coefficient, and hence more tail downforce power, at low speeds and there is a drag penalty to take depending on how much camber.  The Zenith STOL family are the extreme example, and being draggy flying lunchboxes anyway, the penalty is minor in relation to the benefit and allows a smaller, lighter tail unit than would be needed otherwise.



Some high performance and transport aircraft have cambered airfoils for the tail but the amount of camber is subtle (a bit more curvature on bottom than top) and you can only tell if you can see the actual airfoil profile, or examine the surface up close.