nonrandom string

I address this to the list at large hoping that one or more of you can explain something unusual that I observed. A friend of mine and I have been using PGP for several months and I just recently noted that in at least three of the recent posts to me that the first 18 characters of the encrypted message were identical. The first words of the plaintexts were not identical and so I assume that these characters are something else, perhaps the stuffer that I have heard mention of on this list. This repeating string is of concern for obvious reasons (e.g. so much for anonymity), and I would like to understand the cause of its recurrence. Please post to me and I will post a precis of the most reasonable suggestions. cjl