Well, there are various reasons involved, such as follows:
1. Number Of Visitors To Your Blog
It is obvious that you will get more visitors one day and much less another day. As a result, your CTR will decrease. The reason is that due to low traffic, you will get less relevant ads and hence, people will click less on those ads, decreasing CTR.
2. Topic Of Your Blog
The topic of your blog also plays an important factor in deciding the CTR. Technology blogs and blogs related to blogging tips get the maximum CTR while blogs related to other topics get less CTR.
So if making money is your primary aim in blogging, then choose your blog niche wisely.
3. Location Of Visitors
This is also a major factor in deciding CTR. It is seen that CTR will be more if the people who visit your site come from more developed countries. However, you simply can't control this factor, so its better not to think about it too much.
4. Seasonal Variations
CTR varies from season to season. This is because people tend to buy more, hence click more ads during the festive season. In fact, several bloggers report higher CTR on weekends. Some find that holidays or the after period (eg. Chirstmas) can also increase CTR.
5. Proper Ad Placement
You should properly optimize your ad placement and choose the correct ad format and layout, otherwise you might get CTR less than what you expect.