Common nouns varied more. If the accent was on the -ar- syllable, we have generally retained that pronunciation, and almost always spelling has changed to fit it:

• Middle English yerd (yard, garden), pronounced yaird with rolled r, became pronounced and spelled yard;
  
• ME herte (heart), pronounced hairtuh, became pronounced hart and spelled with an added a, heart.
  
• a notable counter-example is ME clerk (cleric, scholar), pronounced clairk, became pronounced clark but its spelling remained unchanged just for the hell of it.

Why Americans pronounce Berkshire, Derby, and clerk as they are written derives from an early-American concern with standards of proper spelling and spelling-based pronunciation. See Daniel J. Boorstin, The Americans - The Colonial Experience, ch 43, Culture by the Book: The Spelling Fetish :

Emphasis on "rules" of proper speaking and writing profoundly influenced the whole American attitude toward pronunciation. It explains what is still the most important distinction between English and American pronunciation, the tendency toward "spelling pronunciation". Very early, Americans began trying to discover how a word "ought" to be pronounced by seeing how it was spelled. This seemed to provide a ready standard of pronunciation in a land without a cultural capital or a ruling intellectual aristocracy.

<...>

Our insistent spelling-pronunciation shows itself in our habit of preserving the full value of syllables.  In long words like secretary, explanatory <...> we preserve the full value of all <...> while the English <...> say "secret'ry", "explanat'ry". <...> Our deference to spelling as a guide to pronunciation has been so strong that we have kept alive here ways of speech which soon died in England. <...> Our weakness for spelling-pronunciation affected the pronunciation of proper names, and especially the names of places. In England these had a purely traditional and casual pronunciation, but Americans who hear Worcester pronounced Wooster are apt to spell it that way; and Birmingham is fully and carefully pronounced, never in the elided English manner.

Birmingham or Nottingham...