There's a noticeable bias to using 10,11, or 12 as either the 1st and 2nd or the 3rd and 4th digits too, especially where the other two digits are lower. Like 11XX, or XX12. Wonder if there's a conscious reason for that or just a notable unconscious human bias for some number combinations?
Maybe, but you would expect there to be about as many people with September birthdays to be using 09 as those with October birthdays using 10, I would think. But there is a very significant gap in their frequencies.
Some portion of the January to September people might pick 9xxy instead of 09xx. (With xx being a day or year.) But can't do that with October to December.