Actually, I take my previous comment back. The last letter of the Greek alphabet is not the letter s.

I'm surprise no one is looking at the Greek root of this. The last letter of the Greek alphabet is the letter s. So, the usage of the letter s instead of the letter z is of Greek origin.

I was taught English the old way. I learned that the suffix ess was only use on titles, not profession. For example, baron is a title for a British nobleman. Baroness would be the feminine form. Mistress is the feminine form of master. Duchess is the feminine form of duke. Empress is the feminine form of emperor. There are few exceptions, such as lion and lioness. I suppose the modern use of ess is an attitude towards feminism . I could be right, or wrong.