When I look at these sorts of diagrams, it seems as if the colours don't blend continuously. Generally I see three rays extending out from the white point that seem much whiter than the surrounding colours, and also a line perpendicular to the "purple" edge which is darker. What's up with that?
One thing to keep in mind is that the colors on these diagrams are only approximations of what the model is actually representing: even if the colorspace of the image can cover it (which is already unlikely), even very good screens cannot cover it completely. (that said, I'm not sure if that's the cause of the perceptual phenomenon you describe, which I also see on some of them).
