The original loot system just pulled a set of affixes randomly from a list where each affix had an equal probability of occurring. The current "smart" loot system just weights certain affixes (ex: primary stat, vitality) heavier than others. The mystic appears to use the same "smart" selection mechanism, so if you're attempting to reroll away from one of the higher weight affixes then it'll occur in the list at a higher frequency.
In other words, "random" does not imply uniform, and the "smart" system was made smart by skewing the distribution away from uniform. The end result is a more painful rerolling process.
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In other words, "random" does not imply uniform, and the "smart" system was made smart by skewing the distribution away from uniform. The end result is a more painful rerolling process.