In Henry Fielding’s Tom Jones, Squire Western and Squire Allworthy are men of the same social class, the same age, and born and raised on neighboring estates. Yet Allworthy speaks the 18th-Century equivalent of BBC English, while Western speaks a broad provincial dialect (which his daughter Sophia seems, inexplicably, never to have picked up). Why is that?
My guess would be that they attended different schools, and had different upbringings, with Western obviously mcuh more interested in the rustic life, while Allworthy was more interested in being a gentleman, which would indicate different educational goals in their parents. Allworthy could also have learned to speak that way so that he is not taken for a rube.