Google is indeed fickle, and rarely forthcoming as to it's hows and why. there are numerous discussions on many forums and no one can say with any certainty what is actually going on. If you are going to actively pursue Adsense, then it can be hit and miss. Like you said, sometimes it may not be worth the effort. Common beliefs says that if you get a lot of clicks, but they do not convert well, Google rewards/compensates accordingly. From what I've read,only things constant with Google is change.
