Best Glide with Flaps T/O in DA20-A1
The best glide flap setting is a matter of definition. If the base airfoil was optimized for high lift (like, say, the Wortmann FX63-137), the cruise flap setting would be some trailing-edge up position. If that is defined as 0°, and the take-off setting is the one for which the airfoil has originally been optimized, then it should not be too surprising that it allows the best L/D.

A high zero-lift drag coefficient, like from a fixed landing gear, will drive the optimum L/D to low speeds and a high lift coefficient. That is where the Wortmann FX63-137 excels and the reason why the optimum L/D of the Katana is at such a low speed. The Katana has inherited this airfoil from the H36 Dimona motorglider, and here it had been selected to meet the minimum sink speed requirement at that time for certification as a motorglider. 

Airfoil trivia: The Wortmann FX63-137 has originally been designed for a human powered aircraft project. This explains its very high maximum lift at low Reynolds numbers.