All kinds of attributes are tied to your challenge rating. Masteries like precision and cruelty increase crit rating by a fixed amount. If your champion has a low base critical rating, the masteries will disproportionally increase crit chance, inflating the index. With every rank-up your critical rating increases, but the mastery stays the same, decreasing its impact. It becomes noticeable with 6*, one of the reasons why some players take away one mastery point from either precision or cruelty mastery.
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