...Productivity index per hour worked. If EU average is 100 than:
Greece stays at 76.3
Germany stays at 123.7 i.e an average German working hour is 62.12% more efficient than an average Greek working hour...
If I may quote your own self: "It doesn't quite work like that."
Productivity is the amount of fiat earned per unit of work. This is a deceptive statistic as the infrastructure in say, Germany is far more advanced than in Greece. A German office worker can type faster on a computer than a Greek office worker can write using pencil and paper. How much of this infrastructure is implicitly included in your 123-76 advantage? How did Germany gain that advantage in the first place?
Imagine 2 identical twins separated at birth 20 years ago. One flies to Berlin and the other to Athens. They both become machinists. The German twin has a higher productivity than his Greek sibling in 2015. Is the Greek kid lazy?
EDIT
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Now they do a house swap. Is the "original German" kid now the lazy one?
The "original German" would teach them how to work more efficiently
Hint: Ferrari was always a great Formula 1 car. Italians are also a Mediterranean nation. Ferrari had problems (10 years without world championship) until Niki Lauda arrived and turned them around. He leaves and soon-after they went nowhere. 15 years without the world championship in, Michael Schumacher arrived and turned them around in the most spectacular way. He leaves and soon-after Ferrari slumps yet again. Sebastian Vettel arrived and is already turning them around. (only this time he competes against Mercedes

Kidding aside, it seems to me you have your conclusion and than construct a premise to support it, a very clever premise granted, but it might lead to a circular argument which is not my forte. While Tante is correct, "people are people" there's nothing wrong in stating believing that some nations are, in average, naturally better in
certain activities than the others, like Kenyan runners being better in marathon than Norwegians while you'd be hard pressed to prove that Norwegians are not better sailors than Kenyans.
This is not, I hope you will grant me that, a stereotype, but acknowledgement of the fact that the climate, geography, history makes those differences ..