I'd never thought about how that s got there, but now I see that that genitive s is just a grammatical sibling of the possessive 's in Mary's house.

我自己则从未想过词尾上怎么会带上了这个s，不过现在我知道了这个属格的s跟玛丽的房子(Marys house)里的所有格的s在语法上都是亲兄弟。