I don't know if it's the case in most languages and cultures but this whole question is quite a delicate one:
'miss' means you're unmarried, but that generally means you're young so it's kind of embarrassing for some ladies to stay 'miss' into their old age, just because they've never married. Don't forget that for centuries getting married and having a family was practically a sign of succeeding in life for many women!
'ms' is relatively recent and is used to avoid giving detail on your marital status. You can be a 'Ms' at any age, even if you're married.