Recently I saw a huge difference between my estimated earning and finalized AdSense earnings (original). So I started searching about and find the possible reasons for it. One reason cached my attention was visitor quality. If your visitor quality is not good or high, you will see huge difference in estimated earnings and finalized AdSense earnings. And the fund will be returned to advertiser. I am using MGID referral widget that can be a case.
I also found about invalid clicks, if you see invalid click activity on your blog you can report here.
You can find many Google AdSense related tips that you probably not know.
I use Google custom search on my blog, it is better that default search. And I was using this for my own search too. But now I know Google strongly suggest us to not to use Google custom search your own search. You can use default search or Google.com, now you can use search bar in WordPress 3.1. But using custom search for your own search is not recommended. There are some reasons for it. Google would not get real data or statistics about search.