Spocket, this might not be of any help at all, but it might be the case with more than just myself... I reincarnated (as far as I feel) into the same country again in a similar culture but with a few key differences, so that I could hopefully work out some major issues from the relevant past life. I wouldn't have consciously remembered other past lives if it weren't for that particular one being so prominent and my trailing my memories back from that. So, maybe in some cases it's that people incarnate into (with memories-- because it's harder to quantify this at all if you consider that many people may have had lots of lives they don't remember) the same culture to finish something. If they didn't have memories, or any issues to resolve, perhaps they might go anywhere, to any other culture, but then those statistics wouldn't show up in the memories people report.
If you're talking about just people who post on this forum, I wouldn't call that a particularly accurate representation of past-life memories in general anyway. Then, besides that, there's no need I can see to slot reincarnating souls into population statistics on this planet. I think that relates closely to the problem of trying to organize reincarnation in terms of population growth. It goes both ways. Besides, in reading through this forum I find a lot of Indian and Chinese past lives. I see many things posted about frequently that seem to be brought into question quite often, and I don't know how they're so easily missed. I will admit I am not familiar myself with any statistical/population studies on reincarnation aside from what's mentioned on this forum (I haven't looked into any of those studies). I do know that statistics can be misleading; cause and effect is very often not what it seems.