I explained for the Zethuka items (here). With new characters and/or a new account, this systematically increased the chance for these items. I had compared this to Nighfall's treasure chest, which can also drop better items, the longer you wait before re-opening them, or also opening them with a new character/account.
I think the elements that affect this are multiple in the game's source code (only 1 could be manipulated, it's not possible if the factors are multiple and numerous).
But it's clear to me that the “virgin” + time factor plays a “reset” role, or at least allows for more luck, apart from other multiple factors.
(I remember a few years ago, every day, over a period of 2-3 weeks, after 5-10 runs in Bogroot, my 3 accounts would drop a Froggy. And in the same order for the drop (account 1, then 2, then 3, etc). Which is a lot of Froggy and contradicts the logic above. But with the luck factor, why not? The only “new” element was an old account donated by an old player, whose new character, who had done nothing in the game and was not killing anyone, was simply opening the chest with group (3 accounts and heros). And I've also noticed it with my low LvL, which prevents any XP gain in the game, with chests on zones. The Terrorweb Dryders in Gate of Pain drop Ghostly staff and Eternal shield q8, but after a while (5-10 runs) this is very very very very rare, especially the Ghostly staff).