It's a common problem with the Color Difference keying method. Keylight does the same thing. I frequently use just the matte from Delta, combining it with the plate in a MatteControl and using MatteControl's own superior spill suppression.
It comes from the max() comparison between channels. Where green is higher than a certain threshold, it gets replaced with the replace color. But noise in the image can push some pixels over the threshold while neighboring pixels keep their color. You can try playing with the Reference color to try to smooth it out, but MatteControl is usually faster and better at suppression.