Apology for silence

While driving the other morning, I noticed the kids were really quiet in the back seat. I was hoping they might actually fall asleep so chose to not turn on their music tape (which we usually listen to while driving). The silence didn't last long.
Amy: Mommy, I'm so sorry I haven't said anything.
Me (totally confused): Why in the world are you apologizing?
Amy: Because I'm sorry I have been so quiet?