The related question and associated answers indicates that the opportunity costs of attunement can be significant driving factor for the purpose of determining a magic item's rarity. Presumably, if you don't have to attune to an item, then you can benefit from it with minimal opportunity cost.
This logic seems to break down when evaluating the Ring of Warmth and the Ring of Resistance (cold):
Ring of Warmth
Ring, uncommon (requires attunement)
While wearing this ring, you have resistance to cold damage. In addition, you and everything you wear and carry are unharmed by temperatures as low as −50 degrees Fahrenheit.
Ring of Resistance
Ring, rare (requires attunement)
You have resistance to one damage type while wearing this ring. The gem in the ring indicates the type, which the GM chooses or determines randomly.
Both rings require attunement, however, the uncommon Ring of Warmth appears to confer all the bonuses of the rare Ring of Resistance and confers bonuses beyond as well.
Is there a reason why the Ring of Warmth is uncommon while a Ring of Cold Resistance would be rare?