No, it is not matter of old or new credits.
In this page: newbielink:http://www.istockphoto.com/sell-stock-photos-exclusivity.php [nonactive] , read this:
"Note: The above examples are based on the gross credit value of $1.52 USD. Royalties above are in USD. Actual royalties are based on net credit value which varies with purchase."
Your royalties depend on how many credits the customer bought.