You might have seen [Invalid activity] notices on your Google Adsense payment history from May/May 2015. Google has adjusted daily earnings in time for a short while, so invalid clicks and exposures are disregarded before the end of the day. they are doing still do some post-processing on activity which can see your monthly balances reduced more, and that they have begun noting these adjustments on your payment report. 

Google has a support article about invalid activity here and what you could do to prevent it. Even legitimate publishers have seen small adjustments to earnings,even after a payment has been issued for the month. this could result in a small negative Current Balance. 

It's very laborious to work out what is this invalid activity, however is also things like invalid clicks, or traffic received from illicit sources and more. If you're a spammer or transmitter or somebody attempting to grind page rank, or have paid for your website to be advertised on a site with by artificial means inflated rank, then don’t be surprised if your earnings are seeing a lot of adjustments. 

Honest publishers don't have anything to fear. With the big variety of traffic flow around the web or internet, a little amount will reach legitimate sites from dangerous sources. Keeping traffic and click activity honest is certainly within the best interests of everyone concerned, advertisers and publishers alike. the difficulty comes with having to somewhat blindly trust that Google implements its policy (omit) and that your website won’t suddenly become subject to large adjustments that you can’t work out. And if you do ever notice unusual activity – like 20 times as several clicks as a normal day, despite no meaningful increase in traffic. this may help prevent you from being deemed an illegitimate publisher, ultimately having your account disabled.