netmeg - 3:48 pm on May 6, 2013 (gmt 0)
That's probably close to it. I believe that some clicks may take longer to validate than others. For example, if they're dealing with a potentially dicey advertiser (I had some clicks removed one time because the advertiser was doing something against against AdWords TOS) it may take more than an instant to investigate that advertiser and his ad campaigns on all sites to determine whether or not it's okay. Or if they see sudden unusual patterns on a particular site or even a particular ad unit, they might want to investigate further or even do a manual review. Who knows? We probably won't, anytime soon.