Adsense pays more in the higher up you have units that are allowed to (and including) 3 Adsense units on one page and many webmasters / bloggers used all 3. However, because all focus on the same keywords (using the same algorithm to find contextual ads) then the units at the bottom of your page will pay less per click than that – purely because users pay more for the # 1 place The in # 10.
Let me explain using an example. The sample of adsense on the right you can see a sample site available WEB.com. They use 3 Adsense units on page 2 and 1 on the right below the content. The unit now assuming # 3 get the most clicks, the webmaster of this site will be scratching your head that wonders why he gets such quantities of waste of money with so many clicks.
The reason is that the # 1 search term (which pays the most) will be placed on the first unit on the page. This unit is in the right first. Unit 1 is likely to have 4 items in it, to be the first 4 keywords that pay off. The number of unit 2 are likely to have another 4 to 8 is the top paying keywords in the list. So when you get to the bottom unit (# 3) you are at 9, 10, 11 and 12 highest paying keywords for your content so unless you talk to lawyers or revolutionary pharmaceutical companies then you are CPC (cost per click) is tiny.
Gain = CPC tiny tiny simple formula really. If you have built your site for the most-clicked ad unit that is not the first / single unit on the page then you lose out.
How much difference does it can not screenshot my Adsense, I can not specific about my figures today, but I can give you an idea widely. On this site I have two units – one at the top of the page eCPM is half a page more than 5x that of the unit just above the comments.
To put that in very real terms, whether received 4 clicks to the first unit – that equal approximately $ 2.50. 4 clicks to my lower unit equal to about $ 0.50. Pretty big difference? Can you imagine if I had 400 clicks on each – not quite so small now?
So there you have it. If one of your Adsense units get significantly more clicks than the other, but underneath it (depending on order page), then you should seriously consider, or move the unit does not perform to somewhere below the well, or remove entirely. Doing this could mean that for every $ 1 you earn, you will now be making $ 5. I hope this helps.