It's because they use the Unix "diff file" philosophy.
From: Dennis S. Wish [mailto:dennis.wish(--nospam--at)gte.net]
Sent: Tuesday, May 16, 2000 9:24 AM
Subject: RE: Different Approach to Virus Protection?
The only thing I am sure of here is that there is no change in performance
and that for a 40 gig drive, they would suggest dedicating 4 gigs.