What's the area of the respective ailerons. Maybe the strip ailerons were significantly larger. The other question would be what kind of ARF wing were you using.

I can see why there would be more adverse yaw with barndoor ailerons since the area you're deflecting has a longer moment-arm. The fuselage on the Kadet is kind of large and slab-sided. Perhaps as the barn door aileron deflects and induces yaw the side-slip of the fuselage starts to work against the aileron.