English spelling is far from being phonetic — and commonly-used proper nouns make the problem even more complex. Often the final arbiter is “it looks correct.” Try writing “success” with one trailing “s” and you’ll see what I mean.
This final aspect of spelling — it “looks wrong” — can be a serious challenge when one cannot see. I can spell well in English, and for regular English text, there are always dictionaries and spell-checkers that come to one’s aid. But spelling commonly-used proper nouns that you’ve only heard others pronounce can still pose a challenge when writing them for the first time.