People always ask me why Google AdSense earnings change so much or why there's such a drastic increase or decrease in the click through rate (CTR) or RPM (revenue per page.)
The first thing you have to understand about AdSense is that how much you earn is directly dependent upon how much the Google AdWords advertisers are spending.
The ads that appear on your site are being paid for by the advertisers.
There is an auction that takes place where the advertisers choose a maximum bid. This auction determines what ads show on your site and the cost of the ad. As you can imagine, this can change drastically from day to day or month to month.
The only thing you know is you receive roughly 68% per click, but ad prices change all the time.
Here's another thing...
AdSense earnings are often seasonal -- meaning you may notice lower earnings during Christmas time or the summer months. If the advertisers are spending less, you are going to earn less.
So don't panic if your traffic is roughly the same but you notice a big drop. This is normal. It could be seasonal or just the fact the auction has caused prices to change.
So you should never come to expect a certain amount from AdSense -- ever. In fact, I hope you are not depending on this income because it will change often.