My biggest theory about my spelling-challenged mind is that I don't care. Spelling is easily corrected, especially on a computer, so I don't feel the need to fill my brain with memorized spelling rules. I'm more for higher thought processes like analysis and application. And how's that for rationalization of a weakness?
Do you have a minor weakness you have to keep an eye on in your writing or in your 'other' job? Is spelling easy for you or do you rely on the computer like I do?