Before 0.2.1, the oldest target is chosen by the oldest idle spider. This is a performance improvement as there are usually a lot less spiders than targets. Since the game scans west to east, then south to north, this can lead to the spiders run from north to south all the time. This is usually compensated a few seconds later by spiders switching targets.
Version 0.2.1 will make a fleet of 5 spiders or less always pick the target that is closest to the oldest idle spider.
Future version will allow you to customize the behaviour in the settings (e.g. always pick the closest, pick closest if less than N targets, pick closest if less than N spiders). That allows you to trade run time for functionality.
