My Google Adsense CTR automatically went down soon as I posted "Sunday Bible Message". It's because it's out of the topic. I usualy post about loans, blogging resources, or making money which are all quite relevant topics.
CTR is the clickthrough rate. The higher your CTR the higher the cost per click. I added few Google ads to increase my CTR and it does really work. However when your posts are irrelevant, it will give you low CTR, as I've observe.
Oh well, I've always been posting Bible Messages during Sunday. My CTR will increase, if I will post only about the bible but I have no intention of making money from the Bible. It's just my little way of spreading the word of God and it's more important than anything else.
Posting irrelevant topics from opps can also causes low CTR. So it will be good to choose relevant topics when doing opps to have good CTR rate.