I believe that dyslexia is a factor, but some people were never educated well. (I do not pretend to be that well educated myself, by the way…)
When I find someone who cannot grasp my “average” (well, it is to me) vocabulary, and has multiple spelling errors, I wonder about their education. Not that they aren’t intelligent, and they aren’t illiterate, either. But when many of the words of an average novel are not understandable to them, they are missing out. And it’s a terrible shame.