I may be way off, but as an adult I still have simaler issues. I read way above my grade level, yet cannot spell. I can memorize a spelling list, but forge it when I write.
I discover as an adult that when I come across a new word I normally mis pronounce it. If I am told how to say it I will remeber it. I now believe this cam from being a whole word learner. I lerned to read words as a whole rather then phoneticly. I believ a stronger foundation in phonics may have been of help. I also now belive this is a form of dyslexia. It normaly goe un noticed becuae of the high reading abilty. I would have an assesmnet and find out for sure. As an adult, I have to remind my clients not to judge my abilty by my spelling or penmanship but by results. Note I normally have to use spell check on everything I type.