I think it's because Ce'Nedra is descended from the original Dryad who left the Wood to marry into the Borune line as part of the treaty. She still seems to be a full Dryad, although she must be at least partly human (Ran Borune, her father, was human, after all). Maybe that's it? I haven't read the books for a while, so I'd need to go and check.
If someone else knows, please feel free to correct me.
