In surprise sets, the probability is based on the guest. So if there are 2 rewards of 50% weightage each, every time a guest is supposed to receive a reward, he has a 50% chance of receiving either one. So there could be a situation where you see 51/52% of Reward A and 49/48% of Reward B. This is because an exact 50% allocation cannot be achieved in dynamically changing pools (example Sign-Ups) as the pool volume will keep changing.

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