I had an 8 point jump and also struggled with whether or not to write an addendum. At one point, a friend recommended that I make up an excuse about getting sick. In the end, I decided to just own up to not being as prepared as I could be the first time around and how I made up for that on my second test. At the LSAC forum, some schools (Notre Dame and Davis are the two I remember off the top of my head) did specifically say they wanted an explanation when they heard that the jump was 8 pts. I think many times schools expect the two scores to be less than 5 pts apart, and if you don't specify, will tell you not to bother.