Per my understanding:
In 1972 Gary Gygax designs Castle Greyhawk, and Rob Kuntz is one of the primary players therein (along with Ernie Gygax and Terry Kuntz). A year or so later, as a reward for being the first player to "beat" the Castle, Rob is promoted by Gary to co-DM of the campaign and the two of them redesign the Castle together and incorporate many of Rob's levels from his own El Raja Key castle into Greyhawk Castle. As play continues over the next several years and new levels are added to the Castle, some are designed by Gary, some by Rob.
Fast forward ahead 30 years, Gary and Rob sit down together to work on "Castle Zagyg" and decide what of the original material to incorporate into the release version -- some of Gary's levels, some of Rob's levels. Eventually there's a disagreement between them about the direction of the project and Rob drops out. He requests that Gary return to him the levels that he designed so that he can release them in the manner he prefers (such as the upcoming "Bottle City" release, or the rumored El Raja Key book). Gary obliges, and therefore those levels will no longer be appearing in "Castle Zagyg."