fully conforming to human behaviors, there are more births per million in certain months in the USA, and more births per million in other nations, (Australia, New Zealand, South Africa, etc) because of seasonal differences. There are more births per million in certain months in predominantly Muslim or Jewish or Hindu or Buddhist nations than in predominantly Christian nations. Fewer births in some winter in far northern regions, (Sweeden, Norway, etc) than in some summer months. This has to do with the likelihood, (probability) of two people of the opposite sex getting together 9 months earlier to procreate.
Bottom line: the mathematical model is smoothed out to avoid these natural variances due to human customs and habits, or due to seasonal variations of weather. But given a region of similar religious and cultural affiliation in the world, (say Denmark, or Quebec, where there's winter weather about an equal number of nights a year, and where both nations are following a similar religious custom [Christmas, etc.]... the likelihood of births to agglomerate into similar date patterns is reasonable, with occasional outliers more reasonable in those areas than if Denmark is compared to Saudi-Arabia, where both weather and cultural customs are markedly different all year long.
The mathematical model is more statistically accurate when applied to the larger sampling of all 7 billion people on the planet than it is to any one nation, region, climactic and/or cultural area, where normal variances are predicted by one of those latter factors.
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