What surprising fact does the narrator realized he had overlooked about them?

From the text:

In through swinging doors I reached a marble foyer, and stopped at the foot of a long white marble flight of stairs. Although they were old stairs, the worn moons in the middle of each step were not very deep. The marble must be unusually hard. That seemed very likely, only too likely, although with all my thought about these stairs this exceptional hardness had not occurred to me. It was surprising that I had overlooked that, that crucial fact.