You were lucky. Each burial chamber has 3 paths going from the central area sometimes there is solid stone and no path. Sometimes there is just a short corridor with a couple turns that dead end and other times there are long corridors with multiple doors and turns. Those usually have cores in them. If you get a burial chamber with 3 long corridors especially if they have a lower or upper level you can get quite a few cores.

The best I've had is 12 cores in one burial chamber and the worst one had 1 dead end and 2 corridors that turned once and ended in a rock fall with yellow mushrooms and zero cores.