your code is not VBA one, but your question is code independent
slabArea is evaluated as the greatest area out of all region objects found in the block
from what you wrote, those regions (or, at least, the one with the greatest area) are most likely the result of some preceeding subtraction boolean operation on two or more original regions so as that "The area equals the combined area for all the objects in the region."
while holeArea is evaluated as the sum of all entities belonging to two specific layers which are likely to host only "simple" polylines or regions (but Arcs, Circles, Ellipses, Hatches and Splines are eligible for Area property, as well)
you may want to post your dwg to see what's in it