Explanation attempt : while the 20 first kilometers around Notre-Dame are uniformely urbanised, areas beyond are more rural with an agglomeration or two here and here. For instance the 50 km stratum could correspond to Mantes-la-Jolie, the 70km one could be Chartres or Compiègne.
I thought these might be self-reported distances too, but then I remembered apartments aren't built with uniform density around a city. It might just be due to unequal concentrations of population in the suburbs.
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