This article is from the PGP FAQ, by Jeff Licquia email@example.com with numerous contributions by others.
PGP does this because of the "-----BEGIN PGP MESSAGE-----" (and
related) headers it uses to mark the beginning of PGP messages. To
keep it from getting confused, it tacks a "- " to the beginning of
every line in the regular text which has a dash at the start. It
strips the extra dash and space when you check the message's
signature, and writes the original text to the output.