A short thread at WebmasterWorld has confirmation from AdWordsAdvisor that the data you see in AdWords, may not be 100% accurate. I do not want to take this out of context, but here is the issue reported in the thread.
You can have a campaign that is only suppose to run in the content network (i.e. AdSense). The AdWords stats will then show that you have content network impressions and clicks, but it will also show, for that campaign, that you have search impressions and clicks for it. This should not be happening, since those campaigns are set to only run for the content network.
AdWordsAdvisor said that "'I've consulted with the tech team on this, and they're aware of it as an issue that is showing up for a tiny number of advertisers." So it appears that this is a "tiny" issue but it is something they are aware of. They are asking you to email AdWords with more details, if you see it in the future.
Forum discussion at WebmasterWorld.