So suppose a card’s “true” interval (given by FSRS) is 100 days. The fuzz range is [95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105].
Normal fuzz chooses the interval randomly and uniformly, so every interval in that range has the same probability of being chosen.
Smart fuzz makes shorter intervals slightly more likely, bu more importantly, it checks how many due cards you have on each of the future dates corresponding to these intervals, and that affects the probability. For example, if on day 102 you have the lowest number of due cards, then the 102 interval will have the highest probability of being selected.
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