One of the reasons is that old files (doc or docx+Compatibility) opened in 2013 does not display at opening all content.

Example:
When you open BatchProcessDocuments Ver 2.4.dotm in 2013 x64 it displays in header:
- a black text (This template ... Enter)
- a red text (The various ... below)
- an image

After you save it in 2013, it displays more text after the image:
- red text (IMPORTANT ... pairs)
- black text (Enter ... ignored)