We try to match quotes using the same algorithm as for emphasis. So when the quote is followed by a space, it won’t be interpreted as an opener for a matched pair.
When you have an unmatched quote like this, we treat it as a left double quote if it’s a double utoe, and a right single quote if it’s a single uote. That explains the asymmetry. Why do it this way? Well, unmatched left double quotes are used in some styles of punctuation, in fictional dialogue continued by the same character. Unmatched left single quotes, though, are likely apostrophes (and hence right).