To me it seems like the list of styles is originally filled with the styles of the document that you first open after installing the PDF-XChange 2012 Office add-in(s) AND when you go into the Bookmark tab of the PDF-XChange 2012 Office add-in.
After this, additional Custom styles are eventually add, every time that you go into the PDF-XChange 2012-tab, in Word, and click Edit Settings > Bookmark. If, at that time, there are additional styles in the document that are not yet in the list, they will be add.
As long as you do not touch the Bookmark tab, there are no additional styles being add, even when you generate a PDF via the Office add-in, and when there are additional styles in the Word document.
I hope this was helpful.