It may happen. See, one person viewed one of your page and then clicks on an adsense ad and gets into the other site. Then he gets back into your site (by history.back) and clicks another ad. In the second time, if your site was not automatically refreshed it wont count against the impressions. There you have 2 clicks for 1 impressions.
One visitor enter your website and click on two ads. Therefore, you get 1 impression and 2 clicks. Nothing wrong