Masking or Adapting?

I was so happy to hear from my child's teacher that he was doing so great at school—cooperative, empathetic, a leader—even though I was totally confused by this. At home he is defiant, bosses us around, is mean to his brother all the time.

Then, I started reading that this is called "masking" and is a really bad thing and is harmful to kids—that they are faking it to conform, to be accepted—and not able to be their true selves. Now I don't know what to think or do about this.

Being much better behaved at school than at home is a very common phenomenon for the kids in my practice. Why does this happen?