This is what I understand about how Necromancer operates, so errors are likely. There are essentially two people in Necromancer, the owners who are Bill and Clark. They have a pool of talents that number around 10 people, they are the ones who have written the modules, aside from Bill and Clark, the Tome of Horrors, do the Cartography and layout.
Then, after thier talents are done they send the product of to whomever is going to actually do the publication, usually White Wolf or Troll Lord Games. The publishers are the ones who do final layouts and send it off to be printed.
From what I understand this is where most of the product "bottle neck" occurs. White Wolf and Troll Lords only have so much "space" available in which to fit Necromancers products. This is where/when the product schedule becomes "locked in".
There are a number of other considerations/decisons that effect all of this as well, but the "space available" in White Wolfs and Troll Lord Games production departments are the biggest factor in the product release schedule.
Hopefully Orcus/Bill will return home soon and give you a far more accurate explanation.