Agencies converting GPA how consistent?

Its interesting to hear what Stacey had to say about converting the GPA from foreign schools. The main comment that perked my interest was regarding hiring an outside service. How would the outside service be able to come up with a method to convert say a percentage (like venkb had indicated) to a GPA. Does the outside service analyze the performance of students from a particular country and benchmark these performances based on a common platform? If such an analysis is indeed performed, does the outside service consult with the school and reach an agreement on what they find? I'm just curious because there seems to be lack of "common ground" if two candidates use two different outside services and apply to the same school. If someone can elaborate on this it will provide a great deal of insight for a newbie like me.

Thanks in advance!
Cheers!