The additonal 'x' was added cosmetically, to disambiguate form (a person named) "Exon" https://en.wikipedia.org/wiki/Exxon#History https://en.wikipedia.org/wik

The additonal 'x' was added cosmetically, to disambiguate form (a person named) "Exon" https://en.wikipedia.org/wiki/Exxon#History https://en.wikipedia.org/wiki/Exxon#History There's no particular reason to believe they wanted to change the pronunciation as well.