Arsenic
To be honest, for first-world countries I'm inclined to put the variation down to simple statistics. Apart from the handful of countries that have a disproportionately large scene for whatever reason (e.g. Israel, UK), I can only name at most a few artists from any given country. Why can I think of three psytrance acts from Macedonia but none from Kosovo? Probably because that's exactly the kind of thing one would expect when the number of artists I know divided by the number of countries I know is of the order 1. Frankly it would be more surprising if the number of artists in each country were "too uniform"; there's some actual maths about this kind of thing which I either don't know or have long since forgotten, but the upshot is that a small sample size leads to a large variation from expected values.