Can children with ADHD behave totally differently at home to in public?

A child I know has been diagnosed with ADHD and to me, he is the quietest, kindess, little mouse I've ever seen, yet the paed has prescribed Ritalin because he is cruel, aggressive and out of control at home.

I had the preconceived idea that ADHD wouldn't be selective, but would also be obvious at the shops, school, park, etc.

Any experiences?