It must be something else, like lighter weight, for example.
From purely efficiency-point of view, a smooth surface without rib ridges is clearly more efficient, at least in free flight. But maybe our free flight airfoils have more theoretical work behind them, and the more accurately you can build the airfoil, more probably it is that it works as calculated.
A better example may be the free flight stabs; A structure similar to I-beam (but with carbon tube spar) has to be 0,4...0,5mm thicker that a D-box stab with "same" airfoil to behave in same way. L