i had a similar question about the Chase model. up till 2005 it was 3975 lbs, in 2006 it goes up to 4843lbs. thats over a 20% increase. same hull dimensions. im not aware of any major change in those years, and certainly not something that would add almost 900 lbs. i called GW and asked. they actually werent 100% sure, but they think at least part of it was possibly including commonly ordered options among other things.
btw, the chase hull was modified to accommodate 4 stroke engines in 2000, but the weight change occurred several years later. im wondering if they didnt re-weigh the boat for a couple years and that is part of it as well
EDIT - just did some more digging around and the hull draft pre and post weight change for my Chase is the same (15 inches). if the hull dimensions are the same and the weight went up 20%, the hull draft would have to change as well.
EDIT 2 - the older manuals on the GW website are scans of physically printed out paper, you can see the three hole punch in the pages. the 1999 and 2000 are the same scan even though this is when the running surface was changed for the 4s.