A WebmasterWorld thread referencing a Google AdSense Help thread where a Googler said they have made adjustments to the AdSense invalid click filter to better block clicks from a specific third party crawler.
A couple weeks ago, Andrew from the Google AdSense team said:
This is Andrew from the Ad Traffic Quality Team at Google. We're aware of a problem with a third-party crawler in the network that is generating invalid activity across a number of different publishers. We've recently made some changes to our filtering so that this invalid activity will be excluded more quickly from your earnings reports and the discrepancy between estimated and actual earnings will be smaller. Robotvsbadger, I'll reach out to you separately via e-mail to check in on the problem.
Two main points here:
(1) Google is aware of a network that is generating nice amounts of invalid clicks on AdSense ads and they are going to or already have stopped it.
(2) Google will remove invalid clicks from estimated earnings reports more quickly so the difference between the estimated and actual will be smaller.
So you may have noticed earnings changed recently do to this.