Si ça peut vous aider les gars, j'avais posé la question en février 2007, et voilà la réponse du staff : "There are various credits in the system which pay different royalties. New credits - credits purchased on/after Jan. 4, 2007 - pay based on the net value of the credit. Please see the link below to determine the net credit value for each of the credit bundles.
http://www.istockphoto.com/pricing2007.php
Credits purchased before Jan. 4, 2007 pay based on a credit value equally US $1. Therefore, it's possible to see different royalty values for a particular image.
Here's a forum thread which explains it better. http://www.istockphoto.com/forum_messages.php?threadid=45268&page=1"