Metathesis is not otherwise productive in the language. Note that all words with metathesis involve numerals. As they point out, numerals form a closed set of words. As a result, the impact on word recognition may be minimized, as compared to the same type of word-initial metathesis occurring among a larger, open set of words. Word frequency may also be relevant.
Numbers are fairly frequent lexical items and so are typically more prone to modification.
University of California Publications in Linguistics. University of California Press. Berkeley and Los Angeles. Kenstowicz, M and C. Kisseberth 1979. Mielke, Jeff and Elizabeth Hume.