It's not puzzling, it's by design.

See the article "The rand "virus": or how to insert dummy text into a
document" at:

http://www.mvps.org/word/FAQs/Formatting/DummyText.htm

Please post any further questions or followup to the newsgroups for the
benefit of others who may be interested.  Unsolicited questions forwarded
directly to me will only be answered on a paid consulting basis.

Hope this helps