I think you are simply confused, or Ms-understanding. Miss and Ms. are two different words with different meanings.
In olden days, Miss or Mrs. was used for unmarried and married respectively, because a women’s status (i.e. who owned her) was important, especially in the days before women could vote.
Seriously, though, around the late 60’s and early 70’s, along with bra burning and other women’s lib activities, came the suggestion that the freshly coined Ms. replace the two other choices in all circumstances, since making marital status an important distinction in naming was considered patronizing and demeaning by the lib movement. It also removed the ambiguity - “How do I address this woman? Is she married?”
You may not remember the days when a woman’s correspondence, if she was married, was addressed to “Mrs. John Smith”, to make the point more insulting. (The rule, IIRC, was that when she was widowed it was time to call her “Mrs. Jane Smith”.)
So if someone calls the person “Mrs. Hillary Clinton” they are being either respectful and/or old fashioned, or insulting, depending on the writer’s intent. In the last many decades, it has never been wrong to say “Ms. Hillary Clinton”. The only exception is when the person herself wants to be called Mrs., in which case she will let you know. Such persons are fewer and fewer every day.
It also makes obsolete the old joke:
Doctor: “Well, I have some good news, Mrs. Johnson…”
Ms. Johnson: “It’s Miss Johnson.”
Doctor: “In that case, I have some bad news, Miss Johnson.”