So I never noticed this before, but if you are a big enough publisher Google has a line item for the invalid traffic amount that is taken out of your payment. One account I manage is big account, but in looking back over the last 12 months - the highest amount taken out for invalid traffic was the cost of a large pizza. However, in February this amount was the equivalent of a mortgage payment on a modest home. Quite the increase + it was already a month where earnings before invalid traffic was down 25-30%.
You might want to check the payments section to see this data, but again you will probably only see this line item if you are a earning a pretty decent amount. My other account, much lower earnings has never had a line item for invalid traffic and did not again this month.
If there's a deduction, you'll see it no matter how large or small the account. There's 70 cents worth of invalid traffic on my all but abandoned personal AdSense account that I had before I switched to my business account. Can't get much smaller than that one, there's only one ad running in it (to top it off to $100 so I can close it out) and it doesn't get 1000 impressions in a year.
Might be as simple as whether you have invalid traffic or not as @netmeg pointed out but, I've seen clicks taken back during the day from time to time and there is no such line item in my account. Could this only be recording end of month adjustments?
Interesting to know they list deduction regardless of how small. I say that because my personal site has NEVER in the last 12 months (according to the payment section) had an invalid traffic deduction which I find impossible to believe. Maybe on this site because it's smaller in size they have been deducting invalid traffic clicks daily?
There must be some difference between the "rolling" amounts they take off in (AFAIK) real time and at the end of the month, and the invalid traffic adjustments that show up in the Payments section. I almost never get the latter either, even during my peak months. But I don't know what the difference is.
Oh, one thing - the first time one showed up on my Payments page (it was for a few bucks, less than five) I emailed support to ask about it and was told it was due to a rogue advertiser, meaning either he didn't pay his bill or he hacked an account or somehow otherwise broke the rules. So that's one cause.