It's weird, I just sort of know if a word is spelt correctly or not. I actually quite at knowing the spelling of words but I have to write or type it first though. If you just ask me to say it then I'll have trouble remembering.

I have the same symptom. My mother always thought it was odd that when asked to spell things out loud that I didn't spell in syllables, but I'm basically reading what I've written down on the virtual piece of paper in my head, so syllables are irrelevant.

I've always figured it was some sort of idiot savant type of thing.

At the same time, there is a small subset of words that I can never remember how to spell correctly, no matter how hard I try. Most of them have to do with sets of double consonants and which ones are doubled. ``Necessary'' vs. ``neccesary'' is probably the biggest one.