EPUB -> EPUB Conversion Results In BOLD Text

I've been using Calibre for many years now, and I've just run into a head-scratcher. For me, anyway.

I have an EPUB text -- an ebook that I got at Smashwords. I always run ebooks through Calibre, even EPUBs -- mostly to justify the text when necessary and adjust the margins so that the right margin is a little wider. When I run this one file through Calibre it makes all of the changes I request ... plus the text changes from normal to bold. I've tried various Font options in Calibre but can't find a way to prevent this from happening.

In terms of my skill level, I use a lot of Calibre's options but I've never gotten down and dirty with file formats. I'm a reader, not a coder.

Any suggestions?

Thanks in advance!