epub export creating extra space on some words

Ah, okay. Now I see what you mean by the extra space.

Well when ID converts to fixed layout, it does crazy manipulations to text size and scaling and space between words and parts of words to try to exactly match the text in InDesign/PDF.  My new educated guess 😉 is that there's something funky going on with the markup InDesign is creating for this one line. 

To cut to the chase: If the problem is only happening in these subheads, no matter which Open Type fonts you use, then there is probably something finicky in the formatting (the styling) that InDesign is choking on. Use a test one-page doc that you've copied/pasted the bad boy text to, and experiment exporting to FXL that test doc. I would try applying [Basic Paragraph] and for Char Style, [None] to one of them, then if that exports okay, change the font to Calibri Bold and export that, and if that's okay change the paragraphs to centered, etc. 

You may have applied some sort of tracking of non-standard letter/word spacing, for example, that ID is having trouble with.

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I made a test file with just your title and subhead set in Calibri Bold, centered.  Look at this mess of html markup that InDesign created with just this 2-line bit of text when exporting to FXL. ("Live" tet is colored black)

This is one of the reasons to avoid using FXL export with text-heavy books. If you're having trouble with images in your reflowable, then learn more about getting them to do what you want. Many many ebooks have lots of images, I've created reflowable epubs with 500+ screenshots (a how-to book on Excel) and 1000+ hi res images and colored sidebars (a photo tour of Tuscany). 

AM