A Google Groups thread asks a question I am sure many Google AdWords advertisers have grappled with in the past: how can you have 1 ad impression but 2 clicks?
Google AdWordsPro writes in to inform the users who run into this problem that the data for AdWords might appear to be updated in real time but is not actually updated instantaneously. AdWordsPro advises users to check their statistics the following day to get an accurate assessment of the number of impressions and clicks in one's AdWords campaign.
The most common reason for this is that account statistics are not updated instantaneously, nor at the same rate. For example, clicks are updated into your account stats roughly every hour, while impressions are updated every few hours.
To see the greatest accuracy in your reports, it's best to view a given day's reports on the following day, when the servers have finished reporting the clicks and impressions.
Discussion continues in Google Groups.