It may have been through film warping, causing it to bend and thus go mildly out of focus. If that were the case previous versions should have the same problems, but it's probably too hard to tell with VHS/Laserdisc. Even on the 2004 DVDs its practically invisible. If the negative was scratched in earlier years it may have been "wetprinted", which often causes a subtly blurrier.

This also may have even just happened. Maybe in 1997 the negatives was fine--this release had the least amount of film clean--up I believe--but by 2003 it had warped a little. You don't do wetgate prints in digital restoration usually, especially for a blockbuster.