From the Official Ad-sense Blog page
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adsense.blogspot.com.au...]
Preventing Accidental Clicks " Fast Clicks are not real clicks "
" Not surprisingly, we found super-fast clicks on ads to provide little to no value to advertisers. That’s why we ignore fast clicks that we detect to be accidental immediately upon ad load. Rather than our ads causing surprise low quality clicks, users can continue on uninterrupted.
Edge clicks lack value
If you’ve used a mobile device, you know fat-fingers are a reality of touchscreens: the average fingerpad is roughly 50px large when pressing down.2 When we’re swiping, pinching, and poking our screens, it’s easy to accidentally touch the edge of an ad that appears unexpectedly or is placed too close to tappable controls on your screen. " .......
There is more to read and it makes sense with mobile phones and smaller devices whereby, many people would be accidentally hitting an Ad whilst attempting to watch a video, or browse a website, due to the small screen size..
As the % of mobile traffic increases, so would the % of accidental clicks...
Pretty basic information, though, it makes sense...