martinibuster - 1:52 pm on Oct 30, 2013 (gmt 0) [edited by: martinibuster at 2:51 pm (utc) on Oct 30, 2013]
Which no matter how you put it, it doesn't make sense. If the ads are no longer relevant then why wouldn't the other ad unit be affected?
When it comes to money spent on advertising, everything makes sense. Very few advertisers spend money without justification. You just have to look and it's usually there. Just don't look for answers in the AdSense control panel. You won't find it there.
Staring at your AdSense control panel will NOT give you answers. Maybe some day Google will provide better data for understanding CTR drops, but right now that's a current shortcoming of the AdSense control panel. Do NOT seek answers there, you won't find them. You won't find sense there.
Dig into your analytics and all of the other points I and others raised. Consider doing more homework and THEN it will start making more sense.
Regarding your "other" ad unit, it's in a different spot, right? That's a reason in itself why it wouldn't be affected. An advertiser might be targeting a specific area OR your site visitors might not be reaching the bottom of your pages or they have consumed enough content before they reached those ads. Ads are consumed as content. The location of ads can be thought of as Content Blocks within a web page as content blocks.
What about the size, is it a different size? Different ad sizes show different ads.
We can't do your analysis for you. But we CAN tell you what to do to find the answers. Next step is yours. Do it and report back what you found.
[edited by: martinibuster at 2:51 pm (utc) on Oct 30, 2013]