People's failure to understand distributions and apply them to real life examples has to be one of the biggest failings of education in the UK recently.
The fact that a teacher might put in somewhere approaching 6 hours 'overtime' a night for a limited period of time or the fact that some teachers might do this regularly in no way means that 30-40 hours of extra work per week is anywhere approaching commonplace for the profession.
In the same way that a few teachers doing absolutely f**k all extra work doesn't mean that's common or average either.
Given that teaching unions (which are very unlikely to be an unbiased source) are reporting around 10 which presumably comes from self reporting (which, again, if anything you'd expect to tend a little bit towards exaggeration) and would suggest that around 10 is a reasonable estimate for the average.