Right, but there's other evidence that even with that, their situation was particularly bad. If you go by the excess-death numbers, Belgium is still the 2nd worst in the world. Why them, and not their neighbors?
A small country or state has an higher chance of being an outlier. 11M people live in Belgium, and a lot of +/- 10M inhabitant regions in larger countries have an higher death rate (Paris, NYC, Lombardy, Madrid ...).
If you live in a large country, some highly impacted regions will be offset by less affected regions.