That's using real worked hours (well, estimates of them by the OECD). In Greece's case the official hours are actually not particularly high for people with regular jobs, just the usual 40 hours/week. But the actual worked hours are quite high, because of the prevalence of unpaid overtime in the private sector, and the large number of mom-and-pop shops that can't afford to hire employees, so work the shop themselves from open 'til close. This contrasts with Northern European work culture, which tends to work exactly the official workweek and not a minute more.

I misremembered Belgium, though; it was the Netherlands that's the lowest in the OECD. Here is the list: http://stats.oecd.org/Index.aspx?DatasetCode=ANHRS