Positioning isn't an exact science. And if you were just on the border, it's entirely possible that you could get that result. If I remember correctly, the OS determines the position using cell phone towers and wifi stations for all ambient positioning in the background. It probably (other forum members might know more about this) only activates the GPS when it really has to, such as when an app is in the foreground and requests an exact position (and the user allows it). The GPS is very expensive, batterywise, so the OS tries hard to minimize its' use. So, imagine how crude the position could be using just towers and wifi - and that could explain what happened to you.