Not sure where you're from, but I can at least confirm that in the Netherlands, we're really bad at estimating the amount of time teachers work - erring towards underestimating it.
There are some additional reasons for this. The first is that there's not a 1-on-1 correlation between number of classes to teach and hours of work. It is assumed that teachers spend, IIRC, 30 minutes before a lesson to prepare it. The time it requires in practice, though, strongly depends on your schedule: if you have to teach five different levels of students, that means you're preparing five different lessons. If you teach five different classes, but all of them at the same level, you'll be able to spread the preparation time over all five of them, greatly lowering the average time spent.
The other complicating factor is that there are seasons: summer holiday is relatively calm (you'll have nothing to do for most of the time), whereas exam weeks can be enormously stressful. So while on average a teacher might be working close to a regular, full-time job, it's practically impossible to maintain a 1fte job, since there will regularly be weeks in which the work is simply too overwhelming.
Edit: And one additional point: teachers are really bad at negotiating. They can demand higher wages or fewer working hours, but once push comes to shove, most of them will simply put up with it for fear of harming the students.
There are some additional reasons for this. The first is that there's not a 1-on-1 correlation between number of classes to teach and hours of work. It is assumed that teachers spend, IIRC, 30 minutes before a lesson to prepare it. The time it requires in practice, though, strongly depends on your schedule: if you have to teach five different levels of students, that means you're preparing five different lessons. If you teach five different classes, but all of them at the same level, you'll be able to spread the preparation time over all five of them, greatly lowering the average time spent.
The other complicating factor is that there are seasons: summer holiday is relatively calm (you'll have nothing to do for most of the time), whereas exam weeks can be enormously stressful. So while on average a teacher might be working close to a regular, full-time job, it's practically impossible to maintain a 1fte job, since there will regularly be weeks in which the work is simply too overwhelming.
Edit: And one additional point: teachers are really bad at negotiating. They can demand higher wages or fewer working hours, but once push comes to shove, most of them will simply put up with it for fear of harming the students.