The answer I was provided with was:
The problem seems to do with not setting the font-weight property on some of these classes.
Make sure the styles in the CKEditor all have the font-weight properties to match the style because the CKEditor applies a default font-weight property.
Note, You will need to reapply the Class to see the changes once you make the updates in the default.js
I did that but I also found that in my stylesheet some styles had sizes in pt and some in px, and some of those differences didn't match in the default.js file, so it was really a combination of both. That is what fixed it for me.
I was going to post the solution, but I just barely got it all figured out at 3 this morning, so I'm glad you asked. Hope it helps. There are WAY too many threads on here with no solutions, just opened tickets, so I always try my best to post my solutions because I know how frustrating it is.