The Head of Slytherin House can recognise all the faces on the silver-green table: part of their parents were his "colleagues" in the Dark Lord’s service. But the specific of the Slytherin house that most of the surnames there have never been changed: from year to year, you know whom to expect. Sometimes there are surprises (like Snape), but this till now every new face behind the table, repeat their faces from Snape’s own ex-classmates. And the Black-boy was the only new one in this posh crowd.