Maybe because it's when winter is at its coldest and so the parents do lots of snuggling. That's the only thing I can think of. Then again, Bev was most likely conceived in the northern hemisphere so it would have been summer at the time for her.
How's this for a scary fact... me and my little sis were born 9 months after my mother's birthday. My eldest sister and younger brother were born 9 months after my father's birthday