is because you are sort of isolated, certainly outside the normal footprint area of your neighbors. Someone would have to get near enough to your home to use your wifi. Few routers effectively broadcast usable signal more than a hundred feet or so. Some also have power rating where you can lower the percent of power used for broadcast, to lower the coverage area.
High power lines pick up wireless signals and can carry them along quite a distance in the electic field around them and some of those signals might have been touching your router and still be viable enough to trigger as an attempted connection. That's also why one can take a flourescent tube, plug one end into the ground under them and have it light up. Microwave towers are more obvious.
Unless your router shows these connects as established in the list of users, nothing to worry about.
If you live along a busy road, people with the newer phones that will use any open wifi band may be getting logged into your router.
You really only need to worry if you see someone having a working connection, because they all can "connect" enough to make the attempt to get an IP assigned to them, if they pass the security.