Simply because the way Xara handles NavBars and other Repeating objects, what you change on one page results in changes elsewhere.
Also unless you use Image Filenaming on your pictures, Xara assigns a numeric filename that can be totally different to its previous iteration.
Change a font and the layout across all pages is altered and all the lines of text need to be recalculated.
The list goes on.
The exported render can be different but not always.
If you use Xara Fast Publish then unchanged assets are not uploaded.
If you FTP, you can direct what is deemed to be a change.
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