When I use calibre's epub to azw3 conversion on the downloaded epub for this book:
https://www.gutenberg.org/ebooks/42664
it uses things like "font-size: 1.29167em;" in the CSS. Whereas for this book
https://www.gutenberg.org/ebooks/42259
it uses the usual "font-size: 1em;".
In the conversion settings under Look & Feel I have Disable font size rescaling
unchecked.
What is in the first book that's making it use the "weird" font size?