During a discussion in our department meeting about whether to rename our TESL program a TESOL program (snore…), I learned that TESL is pronounced [tɛsəl] while TESOL is pronounced [tisɑl]. I’m curious as to why the pronunciation of the first vowel changes depending on the second vowel. I can think of two possibilities:
1) To emphasize the presence of the O in TESOL, speakers want a full vowel in the second syllable, which requires putting a secondary stress on the second syllable, which requires making the second syllable a foot, which in turn requires that the first syllable be a foot by itself: (t__)(sɑl). Being a single light syllable, (tɛ) would not make such a good foot, so (ti) is chosen instead.
2) Maybe [tɛsəl] and [tɛsɑl] would sound too similar, so an additional sound difference was introduced to disambiguate.