Zork 2 and 3 actually contain a lot of new material, which is the origin of the tonal shifts. The original mainframe Zork was basically a larger version of Zork

Zork 2 and 3 actually contain a lot of new material, which is the origin of the tonal shifts. The original mainframe Zork was basically a larger version of Zork 1. Zork 2 took most of the rooms that had to be cut for space, weaved them together into a new map, and added a lot of new things as well, including all the sillier plot elements, like the wizard. Zork 3 was almost entirely new, except for the brief endgame which was the original endgame of mainframe Zork.