Why is fickler a word?

Question

I just tried to instinctively write "more fickle" because fickle is polysyllabic and would, by tradition, get the "more" comparative form vs the "er" comparative form. But MS Word dinged me as wrong. This didn't phase me since Word is frequently wrong, but after a quick double check, it turns out that the comparative form of fickle is indeed "fickler". Which just sounds hideous to my ear.

After much research, I've come up blank as to why this word seems to be an exception to the rule. Does anyone have any insight into why "fickler" is even a word and why "more fickle" is seemingly incorrect?

Answer 1

Practical English Usage by Michael Swan (Oxford, 2005) has

two-syllable adjectives

Adjectives ending in -y have -ier and -iest.

happy happier happiest
easy easier easiest

Some other two-syllable adjectives can have -er and -est, especially adjectives ending in an unstressed vowel, /l/ or /ə(r)/.

narrow narrower narrowest
simple simpler simplest
clever cleverer cleverest
quiet quieter quietest

With many two-syllable adjectives (e.g. polite, common), -er/-est and more/most are both possible. With others (including adjectives ending in -ing, -ed, -ful and -less), only more/most is possible. In general, the structure with more/most is becoming more common. To find out the normal comparative and superlative for a particular two-syllable adjective, check in a good dictionary.

Fickle is pronounced with an unstressed final /əl/, so Swan would allow it to qualify for fickler, although as he says, "the structure with more/most is becoming more common."