In blogs generally page view per user is 1 or 2, and people exit through adsense thats why CTR is high. Whereas in portals or forums, Users spend one or two hour per site, views many pages (approx 30 to 40) after that they exit site thats why there CTR are very low. CTR of forums are generally less thatn 1%. CTR is calculated by page impressions, not by unique visitors.