There is a simple answer to that: No matter how much editing a person does, something always seems to be missed. I learned that with my first novel, Templar: Order and Chaos. I am lucky to have a relative with an English Degree and she is helping me to fix that for the first novel. As I have little funds to hire an editor, i still have to go it solo for now. I love the novels that have come out because of Amazon and I realize now that I am far from the only writer that has a spelling problem. I don’t hold it against the authors, especially the ones that have little money to hire an editor.
Even with an editor, on more professional novels I can still catch spelling mistakes and little grammar mistakes. I see it everyday at highly visible sights like CNN. I also see the people make comments belittling the editors because of the mistakes that were missed. It upsets me that people go out of their way to point that out; it doesn’t matter how many pairs of eyes sees it, little mistakes can be missed on the passes. You could have an army of editors and something could still be missed.
In the end I just smile when I see spelling mistakes and grammar issues. I don’t hold it against anyone.