Why does Thomas lie about not knowing the girl that arrived?

In Chapter Eight, Thomas notes that the newly arrived girl has dark black hair and very pale skin. "She is beautiful, for a dead girl", thinks Thomas. When Alby asks if Thomas knows her, Thomas says he doesn't, and Alby asks him again. Alby's repeating the question causes Thomas to doubt himself. He wonders if he had something to do with the girl's arrival.... if he knew her before, but this is not necessarily a lie.... Thomas doesn't actually rememeber.