There was a time when the spell check stopped working for me on this site. I inquired and was given several half hearted responses as to why it stopped. Just went on with life. One day it started to work again. Go figure.
I believe one reason there are misspellings in posts is that there is an auto correct feature. The computer will guess as to what you meant to type and stick that word in the space. Fore example, if you mistype the word "their" by transposing the last two letters (theri), the computer will change it automatically to "there". Most people depend on the alert found in spellchecker and these misspellings slip through.