Here are my thoughts, guesses, and the (not very substantial)small amount of evidence that I have gathered.
The phonetic similarity of the vowels might also have contributed to the development and maintenance of the variation. (SomeThere are a number of other words can be pronounced with either of Greek or Latin origin spelled with "erV" (where "V" is any vowel letter) that show variation between these two vowels, such as feral, sclera, query, inherent, coherent, adherent, hysteria—although in these words the "near" vowel is actually preferred by prescriptivists because the vowel occurs in a stressed penultimate syllable, or before unstressed "i" followed by another vowel—and (atmo)spheric(al), for which most prescriptive sources seem to prefer the short vowel, but the long vowel of "near" seems to be common, probably in large part because of influence from the related noun (atmo)sphere.)