Is it geology? Perhaps majority origin of surface strata by NUTS region? There is a certain amount of concordance between blue on your map and pink on this.
Spoiler Geological map of western europe :
Hmm, interesting. No, that isn't it. Though I can see how geology could have an interesting roundaway influence on the data presented here. I might afterwards look up a twitter thread I remember which made a very interesting similar connection (though on a different topic, so that doesn't help you here).
It seems to be EU vs non-EU, with the caveat that Andorra should be in the EU, if a special case, and Switzerland shouldn't, so...
The data is from eurostat - don't know why I censored that on second thought - so no. They have cooperations with a whole range of countries or rather their statistical bureaus. See also Turkey which is included. What I don't know is why Norway has no data here. Andorra would probably be light blue (just a guess from my side).
Does anybody know if the UK signed a cooperation with Eurostat or will we lose the UK in future such maps?
As for the map, is it related to drinks ?