Like many self-published authors I don't have the resources behind me that the big publishing houses have, ie, book cover design, marketing, editing, proof-reading, etc, so everything is down to me. I don't even ask anyone to proof read any more, since friends tend not to give an honest opinion (because they don't want to hurt my feelings), haven't got the time, don't actually read that much, and so on. So it's not surprising that typos etc slip through the net.
At least I'm not the only one - just read a book recently by a fairly well known, successful author from a large publishing house with a considerable number of errors in it. Not only that, the number of times one particular word was used (and I'm not talking really common words here, like 'and' or 'said') spoilt my enjoyment of the story somewhat. So if the big guns can't get it perfect, please don't expect one-man-bands to!