I highly doubt that the evaluator lowered the score because of your ethnicity, as IELTS is used to evaluate english language proficiency among foreigners. But if you strongly believe that the evaluator made a mistake (which also happens) you can request a re-mark of your test http://idpielts.me/results/request-a-re-mark/
Did you do any practice tests using official Cambridge books? Did you evaluate your writing component based upon the stated criteria? Sometimes points are cut for word limit, not addressing all the questions, improper use of formal/informal tone, disjoint paragraphs, incorrect spelling, poor grammar etc. Then you get extra points for more advanced use of the language.
How did your practice letters and essays compare to the ideal samples? How did you feel you did on the day?
I have seen some other people also suggest this in some other threads, however If you feel your score is not a true reflection of your performance, then request a re-assessment of your test. The re-mark takes about 8 weeks and costs about half the price of a new test. If your score goes up after the re-mark then you will be refunded. Just like you I was suspicious of my score in the writing section initially scored 6.5, but after the re-assessment it went up to 8.0 and I got my money back.
Your comparison is not fair; even though both tests are meant to evaluate your English proficiency, TOEFL assesses it in a US context while the IELTS does it in a more British context. A British friend took the iBT with me a while ago and he was pissed because I got a higher score than him because "his accent makes it hard to understand what he is saying" (actual comment from the reviewer). Having said that, I completely understand your feeling; I also feel the results of my IELTS don't reflect at all my English proficiency in a Canadian context.