I'm pretty sure it is something I have done wrong or have failed to do right.
I vaguely remember reading somewhere that ATI wasn't quite as good as nVidia for SETI and I recall in the lunatics optimizer thingy-muh-callit tutorial for SETI that there were options for the ATI GPUs.
But, of course when I go back to reference the info, I can't remember where to look (gonna start leaving a trail of breadcrumbs behind me).