The click probably was from the same user. I believe that adsense only credits two clicks per visit.
So I monitor my clicks closely. Considering that they are far and few between, I keep an eye on them all of the time. I checked my account 1 time in the morning, there had been 2 pathetic little clicks and X amount of money. Then later there were 3 clicks, and the same amount of money. That number hasn't updated so it's pretty much like that 3rd click didn't pay anything at all. Why does this happen?
Anyway, I've had days where it takes two or even three clicks to earn 1 cent. I'm pretty sure those aren't invalid clicks, so I'm really starting to believe that some clicks are really worth less than a cent, hence it takes a few of them to add up to one cent! This, in my opinion, is really pathetic.
Yea, some days I'll get a click for like $1.29 and others it'll take 10-11 clicks to get that amount.
It depends on your source of traffic that clicks, the ad they click, and if they've clicked your ads before (or anyone elses ads in a short period of time)
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In addition to what everyone else has said, one other factor is the geopolitical source of the click.
Example: Clicks from Iran, in most niches, are worthless.
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On a related note, does anyone have a rough list of countries that pay less? I assume that the US, Japan, EU nations, Canada, Australia, New Zealand etc would pay well, but what about countries in the Americas (Argentina, Colombia, Brazil etc), or other Asian nations (South Korea, Singapore, China etc)