Does it matter at all that the Cessna172 is an entry level single? Putting the hinge in the center of the wing's chord would be expensive. That there is a gap on the bottom between wing and aileron is unavoidable, else the aileron would not articulate "down". As I recall, the aileron is affixed with "butt" surface mounted hinges with a pin that is retained by crimping the hinge barrel. A cheaper manufacture can not be envisaged, are we over thinking this?
Weight and money, the enemy of every Aero design?