Pretty much what @Mithran said.
Because of the flawed implementation of the drop tables for chaos shard pulls, drops were technically better that was originally designed for chaos shard pulls. After much evidence from the community was presented to the devs, the devs decided to formalize the flawed implementation as the “official” drop table rates as those rates had been already used by players in-game for multiple weeks.
The net result is that chalices drop more often than was originally intended and the upper rarity treasures slightly more than intended.
For reference, this is the thread in question where the devs’ conclusions were announced:
That said, the long-term impact of the erroneous rates on the Underworld is minimal. This is likely the unspoken reason why the devs decided to keep the flawed rates in-game. Although the rates for upper rarity treasures were boosted above their designed values, their increase is small enough that high-end treasures are still too rare to be easily obtained via farming.
The intended supplemental method to acquire the upper rarity treasures is via gems in Daily Deal offers (crowns and lamps) or by microtransaction (sacred treasures). This is not going to change in the future. As mirrored in multiple other progression methods (deeds, medals, etc.), their intent is for players to farm these resources at limited rates and then supplement those resources with additional gem and cash purchases. And yes, as a method of monetization, that means setting acquisition rates of these resources via farming to be low enough to encourage players to seek out these purchasing opportunities, however they are presented to players.