Percy couldn't really answer Molly's question because his apology came from a guilty conscious more than anything. Instinctively he felt like he ought to apologize for apologizing, but realizing how circular and foolish that would be, choose instead to shrug, and explain the best he could. "I'm sorry that I brought up a subject that was emotionally surcharged for you. Of course, I should've known how you'd feel."
Obediently, Percy came when his mother summoned, and sat down, internally anxious, which showed upon his facial expression and body language. He tried to smile when Molly spoke fondly of Arthur, but it wasn't convincing. The crux of the matter came when she opened the Pandora's Box of Percy's disturbance. Falling out?
"You could say that, yes. Actually..." Percy took a deep breath before continuing. "... it was quite a bit more than just a falling out. It was more like an explosion, which had me moving out of the Burrow and not speaking to the rest of the family for about three years."