Why are some pages rasterized to images during PDF to XPS conversion?

Q: I am using PDFTron to convert PDF to XPS. I encountered a
problem.The text in PDF in converted into Images in XPS document. The
Text in PDF is selectable and can be copied and pasted into (say)
Notepad. But When looking into the converted XPS document, some pages
are just converted into images(Documents\1\Resources\Images)
A: PDFTron PDF to XPS (or Silverlight) converter can detect certain
PDF features that may not be accurately represented in XPS (e.g. blend
modes specific to PDF, certain features in transparency groups etc.).
In order to 100% guarantee document accuracy the converter reverts to
rasterization. In case of SilverDox publisher, the XPS also contains
text information, so you do not loose the ability to highlight text,
copy/paste, or to index files on the server.

To disable whole page rasterization you can specify '--norender'
option. This will preserve text and vector graphics, but some pages
may not be 100% accurate.