The words in the latter group originally had the vowel indicated by their present spelling followed by consonantal r. Thus firm would have had the vowel of bit, earth the vowel of get, turn the vowel of good, and word the vowel of fork. There is reason to believe that by Shakespeare's time the various vowels had begun to coalesce with the following r to produce a vowel something like our present-day sound of err. Presumably, as time went on, the vowel of err came to be pronounced in southern England with less and less of the r- coloring, until by the close of the eighteenth century the consonantal r had been lost completely. The same development took place with r before the consonants, as in ford, and after vowels in a word-final position, as in far.
Languages evolve and change to fill the needs of the changing society.
There are many differences between American English and British English - like there are many differences in the varities of the language within themselves. But the bottom line remains, that in spite of the differences, the two are varieties of the one and same language - English.