Percy doesn't know how he could have missed seeing Lavender at school.
Of course, she was only thirteen when he left, so perhaps it's not so surprising, after all.
What does surprise him, even now, is that she fell as hard for him as he did for her. After so many years of being ridiculed by his younger brothers for everything he does and everything he is, it seems… unnatural, somehow.
But there it is. She's not the kind of person to lie about something like that, so the only conclusion is that she really does love him.
And he knows for sure and certain that he loves her. Always and forever.