The scans are not the greatest but maybe you can read them. The inter-electrode capacitance and stray capacitance are the enemy. And then comes the Miller Effect.
I will post a link to a good book with this information in it if I can find one.
The half power point is where Xco = Rl. Increases C causes Xc to decrease. Adding parallel capacitance causes C to increase. Stray capacitance and inter element capacitance are at play.