There's all sorts of different methods that can be use to diffuse light. Perhaps Kobo are using a different method. I honestly don't know.
I entirely agree with you that some PW screens will exhibit more colour fringing than others, but I do think it's basically a question of how sensitive or otherwise the user is to the effect. I think my PW is great, but I don't know whether I have a "good" one, or if the colour fringes simply don't bother me. I have seen a couple of others owned by people I work with, and they seemed just the same as mine to my eyes.