This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
Every time you run PGP, a different session key is generated. This
session key is used as the key for IDEA. As a result, the entire
header and body of the message changes. You will never see the same
output twice, no matter how many times you encrypt the same message to
the same address. This adds to the overall security of PGP.