I am not sure how they handle the negotiations. For instance, say neverblue. Either they go to the companies, or the companies come to them, or both.. Or maybe some middle man linking them..
Say Neverblue has "Product X" and they say they will pay you $25 per sale. When you make a sale for Product X, through Neverblue, you get paid $25. However, the company (product x) pays neverblue $35. So Neverblue is getting paid more than what they are paying you.
I could be wrong, I haven't really promoted CPA for a while, nor really cared about how all the underlying things went down.. so long as I got my money. That is just my understanding of how it goes, though I could be wrong..