Hi Cookie. I typically don't go into our processing details, but I've seen a few of these posts before so I figured I'd take a brief moment to give you some insights into our estimate. In short, we err on being conservative. We provided a 9 day window for your approval estimate in late October. Statistically, for your specific case (at this moment in time) there is a 50% chance your case will be approved before this window and a 25% chance it will be approved later (than the window). In short, there is a 75% chance your case will be approved on or before the estimated window.
Further, if you want to get an estimate of how the CSC is processing I-130s currently, I took a look at some of the VJ stats and can see they appear to be processing cases from last (mid) May right now. This reinforces the late October estimate for your case, as you filed late last September.
As others said, things change and all estimates are 'just that'... but if you are a numbers and statistics person, hopefully this is helpful :).