Your description of the test method is incomplete; for the taped closure, are you securing the minor flaps per requirements of Tappi T804? I think the test results are valid but the different is not likely due to the method used to close the major flaps, but rather what’s happening inside the box with the minor flaps. By using adhesive to secure the major and minor flaps together you’re preventing the minor flaps diving down inside the box and squaring it up from inside. If the boxes you’re testing are empty, the vertical panels will tend to buckle inwards as the box deflects. Free floating minor panels shore-up the panels from inside and maintain their vertical load-carrying capabilities longer.