You'll find the same with gorm, which when spoken has two syllables but takes an a in the plural. The epenthetic vowel (the vowel added in speech to separate consonants) is ignored in these syllable counts. It's best to think of "adjectives of one syllable add an a in the plural" as meaning "adjectives that look as if they have one syllable when written down add an a in the plural".
There are lots of rules in Gaelic (as in other languages) based on phonology, e.g. how many syllables, what sound a word starts with/ends with, etc.
But these rules were laid down over 1000 years ago, even though they have changed a bit since. Whilst the strange spelling system can be annoying, here it is useful. Whenever you meet these rules, just go by the written form - ignore how it is pronounced. For example dh counts as a dental (sound made with the tongue on the teeth) even though it isn't any more.