The easiest way to explain it is to put it into human terms - every time someone has unprotected sex, you can't start a 9 month countdown until baby time. Sex =/= fertilization. Same thing with ball pythons - locks don't equal guaranteed hatchlings on a set schedule (not even close!). Things tend to happen when they happen. Once ovulation happens in ball pythons (which is a very obvious visual occurrence), you can start a pretty reliable countdown. Until then, though, your guess is likely as good as anyone else's.
If you haven't seen this already, it's well worth your time to check it out: Breeding Pictorial by Mark Mandic.