In my experience ISPs move subnets around within their networks as needed to serve the areas they have service. Over time they do have to renumber the ranges to adjust for expansion and contraction of the customer base. They have to keep the IP allocations something like 80% utilized or ARIN will start asking for them back.
Now, how did Google find you?
Most likely it is the wireless signal. We even do this in an Enterprise or business network. We track access points by their MAC addresses mostly because that is the least likely thing to change under normal circumstances. So from a 'rogue' AP stand point you can 'know' all the access points in your network by their MAC addresses. When an unknown AP starts sending a signal on 2.4 or 5 Ghz the monitoring software will see it and unless the MAC address has been spoofed it'll show up on the map of the office floor plan with an approximate location identified as a rogue.
Google street view trucks/cars/bikes had wifi sniffers on them and recorded the signals they heard and where they heard them. Two things are important there, a third is a bit unnerving. They got that you had a wireless AP of some kind because it was broadcasting a signal (BSSID or not) and they got the general location of that signal via their own GPS systems on the Google streetview recorder. The third thing they got which is unnerving is the data that was being transmitted wirelessly at the time.
It's likely not that well encrypted because consumer grade devices don't use fantastic encryption methods.
So, it's pretty easy for Google to tell where you are and it really doesn't matter how rural you think you are - if they have a 'street view' view for your street then they know where you are.
The company I work for makes appliances and software that does this sort of location tracking for business wireless networks - it's cool stuff and I like it generally.