It isn't even that he skipped piano practice that the mother minds so much, as that her son lied to her about it. And, by implication, she's not so sure she likes the new creature he is becoming. And he seldom disobeyed when he was younger.
Adolescence is what's going on.
I think the reason the mom we mentioned above was more hurt by the lie than by the disobedience was that on some level she realized that her son had chosen his relationship with his friend over his relationship to her. The lying cost him something in terms of his relationship with his mom. But giving up the opportunity to be with a peer would have, in his scheme of things, cost him far more, and in an area where he is far less certain of his standing. Parents, in other words, get their feelings hurt by their children's not telling them the truth because at bottom the parent realizes it is a sign that her child is pulling away from her, and there is some pain in letting go. It hurts your feelings when your preteen lies to you, but unlike when she was younger, your teenager is not so powerfully motivated to avoid eliciting your anger or disappointment.