If I understood your question, you are asking why the CPC for your targeted keyword is higher in the AdWords Keyword tool than what you are seeing in your AdSense account. There are several reasons for this difference.
First and foremost, you are looking at the wrong data. The AdWords Keyword tool shows you data for the Search Network ads, the ads displayed on your website are from the Display Network, not Search. The CPCs tend to be much lower for the Display Network since those are less targeted ads. Instead, use the Contextual Targeting Tool to get data from the Display Network.
Secondly, your actual CPCs will vary based on how much advertisers are willing to pay for ad space on your specific website. If your visitors establish a reputation for being high converting traffic then advertisers will be willing to bid higher for ad space on your pages, and if your traffic proves to be poor for advertisers you may have little to no bids for ad space on your pages.
Third, if you have more than one ad unit per page then, on average, you will receive less than a top dollar as the addition ad units will lower the average CPC due to showing more ads including ads placed by the lower bidders.
