Is that Google Adsense? It works very simple. Somebody clicks an ad, hits the back button on their browser window, and clicks again.
I believe if you use the <meta> tag "no-cache" then you won't get above 100% ctr since the page is refreshed when hitting back, but nobody uses no-cache therefore it's not counted as another impression.
I know because I have sites that have over 100% ctr without a problem. Btw, websites that target women usually have higher ctr just so you know.