1) Spells such as Fly, Dimension Door, and Choke are disabled on the map - and consequently, all generators that grant them, including artifacts like the Orator's Hat, Book of Air Magic, and Book of Earth Magic.
2) Two relics that you didn't get in your generation are clearly present on the map, and therefore do not participate in the random pool.
1. And I thought that disabling certain spells only affected their drop rate in towns and sanctuaries. So that's how it works?
2. Live and learn. Does this mean that any artifact located on the map will never drop in tombs, Dragon Utopia, or other one-time generators?
your level of play will depend heavily on a "convenient" layout
I don't see the connection. For example, if the Helmet of Divine Enlightenment drops in a warrior's tomb instead of a unicorn mane bowstring—will that somehow affect my skill? Hardly. You might be able to complete the map faster with a lucky layout, and that's all. Some mobs or banks might be broken through earlier. In any case, I will put in every effort I am capable of, but in one scenario it will happen sooner, and in another later. Provided, of course, that the game is played for speed rather than just to complete it.
I fought this several times when a Week of Plague would suddenly drop at the most inconvenient moment; you'd think: exit, enter again, load an autosave, end turn - and something else should drop! But nothing of the sort: time and again, the Week of Plague dropped again!
Show a little more patience and luck will smile upon you :) Sometimes the Plague gives in after the first autosave reload.