This isn't due to the Puppy Cave being special per se; it just has a bit to do with how drops work. Puppy Cave has two high monster intensity levels, 5 and 6, on which the PC will get many more kills than average. In addition, lesser vaults like PC:6 have atypical item drop rates. On average, completing the PC will give a fair number of items simply because you have to kill a lot of monsters to get through. Particular items (eg. spellbooks) are no more common in one dungeon than in any other, however.
I always like checking the Puppy Cave as a caster because I've always had very good luck finding shops there. I might speculate on reasons why there could actually be more shops there, but it's probably just positive bias on my part.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.