I've also found this:
I had an ad that ranked around #5 for a rather popular keyword. Then I had it paused for 3 or 4 months. When I put it back it ranked #1 for that keyword. As I see it, CTR generally decrease over time, as more competitors come to adwords. But my ad's CTR didn't decrease as it was paused.
One possibility is to count the CTR since the Ad is created. If you edit the Ad, the CTR resets. The problem with this would be that an Ad could have a high CTR when it was alone and now have a low CTR as it is together with more interesting Ads. In the short term it would be unfair to new Ads to count the CTR from the beginning. The same could happen if the user had paused his campaign when he had a high CTR, as the case that johannes mentions. If Google was trying to avoid that, then it would reset the CTR when an Ad is paused and reactivated.