Spelling is a kind of talent that some people have, and some don't. In the extremely variable English language, correct spelling requires intensive memorization for anyone without that talent. Even simple words may elude some otherwise quite intelligent people; spelling is a niche skill that doesn't generally reflect a person's general capability.
Three possibilities:
1. You have natural talent for spelling
2. You have natural talent for memorization and use it to help your spelling
3. You spell phonetically and often incorrectly (because English doesn't always follow the rules)
The natural talent for spelling occurs more often in both AS and autism than among NTs, chiefly among visual thinkers who see words in their written form, rather than as mental representations of how they sound. A misspelled word simply "looks wrong" to a visual thinker with this sort of spelling expertise. I have this talent myself; I did not learn to read by phonics, but by the appearance of entire words. If I'm exposed to a word once (or twice if it doesn't follow the rules), I know what it looks like and thus how to spell it. That's not truly a commendable thing, since I had to do no work at all to come by this ability.
Aspies without said talent are more likely to be good at spelling than NTs because they do have that capacity for memorization; but not all Aspies are interested enough in spelling to make the effort. Quite a few people on the spectrum read dictionaries, though; and that's definitely conducive to good spelling.
But if you have natural talent for neither spelling nor memorization--these talents are common but certainly not omnipresent on the Spectrum--then you are forced to spell by the rules you know about words--"I before E" and so forth, as well as the rules of phonics and any patterns you've noticed. Because the English language has no hard-and-fast rules about spelling, even after Webster's overhaul, this means that mistakes will be made.
I think that Aspies may be more likely to be memorizers, and Auties more likely to be naturally good spellers; but then, I myself am an exception to that rule--an Aspie with natural talent. However, the lack of spelling expertise says nothing about whether or not a person is on the Spectrum, or which part of it he is on (barring LFA with comorbid MR, which can of course result in illiteracy).