I think what makes people question the data is that the average hours of work per week match up pretty well with the time a teacher spends at school, which is insufficient to cover duties like planning lessons and grading papers (see for example any work-to-contract strike).

In any case, the ATUS specifies that its questions are about work done for pay/profit or free for a family business (since teachers typically work more hours than stated in their contract, a lot of work they do is not for pay).