In the history of English phonology, there have been many diachronic sound changes affecting vowels, especially involving phonemic splits and mergers.
The shortening of ante-penultimate syllables in Middle English created many long–short pairs.
[1] Some varieties (including most American English dialects) have significant vocalic neutralization before intervocalic /r/, as well.
Kenstowicz (1994) states that "... American English schwa deletes in medial posttonic syllables ...", and gives as examples words such as sep(a)rate (as an adjective), choc(o)late, cam(e)ra and elab(o)rate (as an adjective), where the schwa (represented by the letters in parentheses) has a tendency to be deleted.
Various mergers before historic coda r are very common in English dialects.