Matt had that paper and it just kept bugging him. He wasn't the adopted kid that wanted to know his true origins, like where he came from and all that. He was put in the system from a very young age so being adopted was never a problem. They were his real parents as much as he was concerned. Then Brendon came along. He never really thought about what he said until that woman came. Now his curiosity was being scratched. At least it wouldn't hurt to know. Well it wouldn't hurt him but his mum.
He might have used the 'you're not my real parents' card, but he was just being a brat as usual. It was just a lot of things were happening at the time and he had lashed out so…the paper was bugging him. He hadn't really meant he'd call her when he told his mum. He just said it out of spite you know. To irk her.