I thought so, too. But when we have visited places, Brother Genitivi has been correct about them.

[ And then, clearly reading a page: ]

"What is a matter of choice for most human folk is dictated entirely by birth for dwarves. No one may become a smith who was not born to Smith Caste parents. A servant who marries a noblewoman will never be a noble himself, and although his daughters would be nobles, his sons would be servants, for daughters inherit the caste of their mother, while sons inherit the caste of their father."

I think the dwarves who do not wish to be part of their caste go to the surface.