Or it obviously mean 2-99, 100-999, 1000-9999 and so on...
so, it's not linked to population (San Marino has less than a million inhabitants, probably, I didn't look it up), it's not percentage. Might it be related to medicine (they have those x in y-numbers there)?
And it seems to be that low is better than high, so it might be deaths from x? But then the numbers would have to be from a long period of time and accumulated (else San Marino...). So not really likely.
Still, it seems to be (negatively) linked with population, as red countries generally seem to have a lower population than blue ones.