I agree with Dagon.
This experiment assumes that the subjective probabilities of participants were identical to the stated probabilities. In reality, I feel like people are probably wary of stated probabilities due to experiences with or fears of shysters and conmen. That, is if asked to choose between 1A and 1B, 1B offers the possibility that the `randomising mechanism' that the experimenter is offering is in fact rigged.
Even if the experimenter is completely honest in their statement of their own subjective probabilities, they may simply disagree with that of the participants. Whatever `randomising mechanism' is suggested is, of course, almost certainly completely predictable given sufficient information - a die roll, or similar, predictable using Newtonian mechanics. That, is the experimenter's stated probability is purely a reflection of their own information concerning that mechanism, which may be completely at odds with the participant's knowledge.