MicrostockGroup

If there's an extended legal guarantee, you get nothing from that, so you'd only have the royalty from the image license itself. Could that be it?

Check out also this thread:
http://www.istockphoto.com/forum_messages.php?threadid=330442&page=1#post6407718
in which a contributor says:
"I had a same "glitch" few days ago when I received only $14.40 for 100 credits EL. Support told that there is a some problem with EL's and they should fix it and pay us the rest."
Strangely, however, the original post has apparently disappeared or 'been disappeared', and I can only see the post quoted in someone else's post.

I received the same exact amount for an EL sold on June 2nd.  Great...yet another glitch to wait on for them to fix.    

They could make a lot more money selling more EL's.  I sell much more with SS, never really understood why istock can't get more of their customers to use EL's.

To be honest, the 'glitch' is your best hope. Other people have been getting the cut and paste 'old credits' answer, others have been getting the 'discounts' answer. At least with 'glitch', there's a good chance you'll get your correct payment eventually.