As can be seen, there is a difference in the 3D mesh, where the
Recombine command generates only one element between any polygon in the 2D mesh, resulting in prisms. Leaving out this command, results in smaller, tetrahedral elements.
This can also be seen in the number of elements produces, shown in the output of the mesh-command:
t3a: 15570 vertices 33542 elements
t3b: 15570 vertices 90994 elements