While your math (on the surface) seems fine, you also aren’t factoring in things (which may/may not alter percentages) such as but not limited to: the chest may not drop at each encounter, some items may be weighted higher/lower for drop chances, and the various rolls an item could have.
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I checked light.gg and each piece of the armor set has a nearly identical rarity among members, so I don't think there's a difference there. But you make some good points!