Adsense is notorious for this phenomenon. There are many reasons, some known and some unknown. The problem is that google is extremely secretive with the adsense and adwords parameters they use.
I've heard the main reason this happens is because the quality of the advertisers is quite low and the quality of the traffic is low as well. In fact, the higher the page rank of your adsense sites is, the more you will earn as your "trust factor" increases.
My adsense per click ranges from 0.00 to 1.80+ per click.
Its all over the map.
You can block the low paying advertisers in through your adsense account. Just google how to do that and that should help you out.
Don't bother emailing google, they DO NOT care and will not reply.