When keeping Cal kings in a large enclosure together over Winter, the big male and big female stayed coiled together much of the time. Something you can't see in racks where animals are kept separate. The female layed eggs that Spring. I have no idea when she became pregnant.
A researcher of many years has seen wild females gravid in Winter (kingsnakes). He theorizes that, depending on conditions that year, there is no reason why females cannot be gravid. I know, this is the Hognose forum, but I can't help but wonder.
Just because we keep snakes according to some cookbook method doesn't mean that we maximize their potential as breeders. Perhaps in the wild, early clutches may be more common under certain conditions, while in our one-box-per-snake-rack systems, it is virtually nonexistent. (BTW, I'm just throwing out ideas to ponder to the masses. I don't know how you keep your snakes persay, and I'm not saying I know how to do it better than you or anyone else.)
What would be really interesting is if you kept some animals in groups in a larger enclosure over the Winter and during Spring to see what would happen. Maybe those infertile clutches you saw in Jan, wouldn't be infertile? Fun to try and see anyway.