Simple. Nikon make their own filters, and use the same integrated multi coating technology and optic glass on their lenses. The filter quality is on par with B+W and Heliopan MRC version counterpart.
The sad story about Canon filter is that it is a re-badged cheap single coated Tiffen so basically you get what you paid for.
in my experience, the canon filters are absolutely terrible. i compared them to B+Ws, hoya green label, hoya S-HMCs, and heliopan standards and the two canons that i had were even worse than the el cheapo hoya green labels. i don't have any experience with the nikon filters but i understand they are very, very good.
