It took him a second to process what she meant. Why had he never prepared an explanation that seemed normal for questions about his scars? That seemed so obvious a thing to do now. "No. A lot of them are, but not all."
Atticus nodded at her explanation. "My family never really had a house alliance, but then my dad's side almost all went to Beauxbatons, so there couldn't be much." Atticus could pick up that there was someone outside the compartment door based on the sound of footsteps, but he had just assumed it was someone walking past until Heather opened the door to the girl beyond.
He watched Heather's reaction pretty closely. It seemed weird to him that there could be such animosity so quickly, but then the girl had done the same to him, at least in looks. He still had no idea what he'd done to tick her off, or why she seemed to care so much about it. In a weird way it almost felt like a payback for the seeming random dislike of himself.
Atticus briefly glanced out the window at her comment, sure that they hadn't even started moving yet. The ride to Hogwarts took hours normally. In his first year, they'd set off in the late morning and only arrived in time for tea, and it definitely hadn't been hours. Maybe she just needed a watch? Maybe she'd been one of the first years who (inevitably, every year) bought so much candy from the trolley witch that they lost track of time altogether. He wasn't blaming her, there were plenty of sensible reasons for any confusion about timings.
"Are you trying to say Slytherin's better or something?" That was certainly how Atticus interpreted it. He didn't try to sound confrontational as he said it. Given he had no idea how big or small whatever he'd done to annoy the girl was (or even if he'd done it) he didn't need to add anything.