There was a thin sheet of plastic buried a few inches below the ground surface that I was unaware of, and the ants had actually built their colony through the plastic. This caused there to be a fairly distinct line where the chambers are closer than usual. You can see in the middle of this picture and it's noticeable at other places around the cast. See the melted plastic on the cast right after pouring here.

Either the plastic provided support for the above chambers, allowing the ants to build the chambers below right up against the plastic or the smoothness of the plastic didn't provide support for the chamber ceilings below that normally would have been held in place by the friction of dirt above. Either way it's an interesting feature.