Trueman - 7:59 pm on Jan 11, 2012 (gmt 0)
I digged a little bit more into this issue. There's an interesting thread where some people claim, they have a discrepancy of up to 40%.
The summary of a Google employee is:
When more invalid clicks are detected, it's not necessarily clear why. It could be a change in SEO quality, user behavior, fraudulent activity, site layout, etc.
Estimated earnings are
like the "estimated readings" your electric company may use to bill you in advance; sometimes they are accurate, sometimes not. Only real meter readings (finalized earnings) accurately reflect the reality.
So forget stats and CTR in Adsense, they can be 40% inaccurate.