I think it's because our brain can't really focus on both content and spelling at the same time. You can only really check either the message or the spelling at one time when you are the author.

When you check the message/content of your post, you look at every sentence and ask yourself: does it convey my point? Did I choose the right words?

When you check spelling, you should check word by word without looking at the meaning(unless spelling depends on it). Since you know what's coming next in your story, you're probably just rushing through the sentences. You'll miss stuff because you don't read every word. It is the classic "the the" problem where the same word is shown twice in a sentence, but you miss it because you only fastread it.

Also, spell check last. If you spellcheck first and then do some rewriting, the new stuff will have a high chance of spelling errors.