dunno where the 1/10th comes from, maybe Poke can tell us...
...but I think that the arguement is that you can't put in place an agreement that forces Mexico (for instance) to keep its current standard of living for its citizens, while permitting Western Europe (for instance) to maintain a wealthy, middle-class lifestyle. You would have to remove the word "developing" from Mexico's designation.
It is a tough problem that I have no solution for.