I display 3 ad units on each page of my site (and 1 link unit), top, left side and bottom, and by using channels, have notced that the ads at the bottom although the CTR is lower, get on average a much higher revenue per click, can anyone one you adsense guru's out there see a reason for this.

I'm considering removing one of the units, and would normally consider removing the bottom placed unit, would that mean the higher paying ads appear elsewhere?

Actually adsense optimization tips suggest having multiple units help earn more but we (or actually I) tend to think one ad unit will have highest paying ad giving me more money. I am now seeing by implementing what they have suggested.

It could be that the bottom ad is placed first in your html code. If that is the case then Google's software probably thinks it is the top ad and not a bottom ad.

I would say that if you have less ads then you will get a few less clicks, but the clicks are likely to earn you more depending on how much competition their is for your niche.

I was going to say the same as John. The highest paying ads should be the ones that show first in your code. If the ones you have at the bottom of the page are the first to appear in the code they should have the highest payout.