Why was 異 (uncommon; to distinguish) adopted to mean the chemistry prefix of iso- (equal)? Doubtless the non-chemical meanings of 異 are  the polar opposite of iso- in chemistry, as in isomers.
Taiwan's National Academy of Education Research and Wiktionary blazon this meaning.
CUHK and ZDIC omitted this chemistry meaning. So did ABC Chinese–English Comprehensive Dictionary (2003), p 1134.  I can't access the &quot;slightly revised through July 2005&quot; ed.
Isomer is &quot;同分异构体&quot;, which briefly written as &quot;异构体&quot;.
&quot;异&quot; emphasizes the &quot;slightly difference&quot; of the arrangement of the atoms.
&quot;同分&quot;(same number/type of atoms, and majorityly same arrangement) is often omitted, so &quot;iso-&quot; is &quot;异(異)-&quot;. For more types there is &quot;新-&quot;.
Some other examples like &quot;顺(順)-&quot; and &quot;反-&quot;: 順-2-戊烯/反-2-戊烯, 順式脂肪/反式脂肪.