CPU utilization factor from 0.8 to 1

I noticed that my GPU (nVidia GTX 260) is now crunching Albert WUs with these parameters:

1.00 CPUs + 1.00 NVIDIA GPUs

Average completion time is 5300s

From the 1st WU till few days ago (don't know exactly when) the utilization factor for CPU was 0.8 and average completion time was the same!

With 0.8 my quadcore was able to crunch 4 WUs on other CPU projects (real % utilization due to GPU was flexible, from 0 to 50% of one core), now it is able to crunch only 3 and no improvement on Albert WU completion time.

Did you changed something? And if yes, why?

P.S. now I changed GPU u.f. to 0.5 to check what happens ;-)