Metathesis also explains why we have three and third rather than three and thrid.
Ok, but we still have thread - so why does metathesis strike in one place but not in another? I think we are onto something very subtle here. BTW, I always thought that phonemes were sound units (rendered in general by several letters) not just vowels or consonants. Each language has familiar and less familiar phonemes. So "adl" in English is an unfamiliar phoneme (it doesn't occur in any genuine word, to my knowledge), whereas e.g. "oodl" or "atl" are familiar ones, which would not be changed to "oold" or "alt".