Statistical Anamoly

Three nephews, 2 god-daughters, 12 months a year. Five birthdays to spoil my favorite kids. Doesn't sound all that complicated right?
So someone explain to me how it is 4 of the 5 have birthdays within 4 weeks of each other, and within 4 weeks of Christmas? So today I wasn't shopping for just Christmas presents. Oh no. I had to get 5 Christmas presents (for kids) and 4 birthday presents.

But did I? No. Instead, I bought zero birthday presents, and 3 Christmas presents. That would be joint presents for 2 girls, and 2 boys. And one kid is getting his very own present. And 3 of them are going to get "special shopping trips" with *erinannie* when they see her this week. And the one kid who had the decency to be born in July is getting a much better present.
(All that being said, I finished all my Christmas shopping today. And to my family that decided we're doing a name draw this year after I had finished my shopping? Well, you're all getting presents anyway. You don't have to give me Christmas presents. But I'm expecting a generous birthday. January 17. Don't forget it.)