Prop Comparison: Gemfan 5045BN vs KingKong 5040 and 6040

I try out everybody's favorite prop, the Gemfan 5045BN, with unexpected results. I'm open to anybody who has suggestions why my results with it were so poor.
My frame is the RedRotor Strider Flex, which has adjustable / folding arms. This is how I was able to easily switch between 6" and 5" props without changing anything else about the setup. Not just the props changed, but also the motor-to-motor distance.