Yes, and it has to do with fashions of royal address. According to Francois Velde of heraldica.org, “Your Highness” was the most common style across Europe from the 12th through 15th centuries CE. When Charles V became Holy Roman Emperor in 1519, he decided that he was too special to be a mere “Highness” – he did after all rule an empire that spanned from Asia to the Americas to eastern Europe – and instructed his court that he be described as “Majesty.”
Francois I of France, who would spend his entire reign fighting Charles V, wasn’t going to take this lying down and so insisted on also being referred to as “Majesty,” which led to a good deal of linguistic one-upmanship where Charles insisted on being called “Imperial Majesty” to Francois’ mere “Royal Majesty” and then Francois retaliated by using “Most Christian Majesty,” and so on. Henry VIII of England, who would be out-ego’d by no man, also began using “Majesty.” Just to confuse everyone, though, the English court continued to call him “Grace” and “Highness” as well as “Majesty” in legal documents, until the reign of James I, where it was regularized that the King is a “Majesty,” and Dukes and Archbishops are “Graces.”
So the TLDR is: a Majesty outranks a Grace.
To answer your second question, this is because GRRM wanted to simplify terms of address, the same reason that he used the same title of “lord” for everyone.
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