The books actually hint that the distribution isn't random at all, after all every family member of harry's, beside his father, that we know off is a magic user. Molly's mother is a magic user, perhaps some of her siblings will show the talent, too.
That seems like wizardry is a hereditary thing which means you will not have an even distribution. I'm gonna bet that there are some crazy-old wizard families in europe or (not so crazy-old but still oldish) in new england.
Based off some of the information available from the novels, as well as comments made by Jim at a few points, here are some things to consider.
Magic tends to be an inherited condition, which usually follows Salic law, which generally means that it is passed along matrilineal lines. Which also raises the interesting question of just whom is Harry's maternal grandmother
From a post by Jim,
here, it is strongly suggested that in order for ability to handle magic to be passed down, the child has to have been conceived/born at a time when the mother/parent still possessed the ability to use magic.
One of the other things which Jim has apparently said previously (not sure where, can't find the citation presently) is that in the past, there might only be one or two White Council-level wizards born in a (European/Western) country in a generation. Given the explosion in the human population in the last century, as well as the medical advances which means many ailments which would often kill infants and children before they would be 'old enough' to manifest magic, it is not too surprising that more and more potential candidates would appear. The current world population is ~6.8 billion, and at the start of 1900, ~1.9 billion, with Europe likely having ~16-32+/- new candidates for membership in the White Council developing in the generation around 1900. What this would translate into now, is Europe (or the US/Canda) likely having something like 56-100+ candidates develope in the current generation, Harry being one of the more notable ones.
As has also been mentioned in the books, the White Council tends to be a very Eurocentric group, with some members coming from the Mideast, Native Americans, India/South Asia and the Far East, (the Gatekeeper, Joe Listens to Winds and Ancient Mai being obvious) but there are apparently other magical people from these cultures that wield significant powers without being members of the White Council. So it is quite possible that areas outside of Europe or former European colonies could have their own distinct magical traditions which Jim just hasn't (or won't?) gotten to.