You may have seen this, or ‘Invalid activity’ notices on your Google Adsense payment history from September/October 2012. Google has adjusted daily earnings in real-time for a while, so that invalid clicks and exposures are disregarded before the end of the day. They do still do some post-processing on activity which may see your monthly balances reduced further, and they have begun noting these adjustments on your payment report.
Google has a support article about invalid activity here and what you can do to prevent it. Even legitimate publishers (such as I) have seen small adjustments to earnings, even after a payment has been issued for the month in question. This can lead to a small negative Current Balance (denoted by brackets around the number). It is very hard to work out what constitutes this invalid activity, but may be things such as invalid clicks, or traffic received from unsavoury or illicit sources. If you are a spammer or someone trying to grind pagerank, or have paid for your site to be advertised on a site with artificially inflated rank, then don’t be surprised if your earnings are seeing lots of adjustments. Honest publishers have nothing to fear. With the wide variety of traffic flow around the internet, a small amount will reach legitimate sites from bad sources. Small, occasional adjustments are nothing to worry about (although I’ll agree it’s disappointing to see earnings revised down!).
Keeping traffic and click activity honest is definitely in the best interests of everybody concerned, advertisers and publishers alike. The trouble comes with having to somewhat blindly trust that Google implements its policy fairly and that your site won’t suddenly become subject to huge adjustments that you can’t work out. I recommend the ‘How to help prevent invalid activity’ section of the support article linked above, and if you do ever notice unusual activity – such as 10 times as many clicks as a normal day, despite no meaningful increase in traffic, the best thing you can do to protect yourself is alert Google proactively. This will help prevent you from being deemed an illegitimate publisher, ultimately having your account disabled.