I'm also going to copy over a similar set of questions I answered previously that might be useful here as well:
- Question: "If you say you took the test more seriously and prepared better for it, that’s the truth, isn’t it?"
Be careful with this explanation because there's a second message you are conveying without realizing it: you didn't take it seriously the first time. I've had a dean or two mention they particularly dislike this explanation because it indicates to them that you might not take law school seriously either since you didn't think to be serious enough about your first LSAT take.
- Question: "should I just say something about an anxiety attack during the first test"
I too have concerns about discussing this in an addendum, especially because that short form doesn't often allow you to talk about how you have learned to mitigate the effects of these over time. My preferred solution for this is to instead mention that there were "issues" during the test that affected your final score. This neatly identifies that there were problems without being specific about the exact nature of those problems.
- Question: "I know a few schools definitely suggest writing addendums for large jumps in their FAQ"
They do, but I think it's important to assess each situation individually. First, some of those notices are legacy comments from times when double-digit increases were less common. These days we see more big increases, and so do the law schools so they are less sensitive to them. Second, what constitutes a "large jump" at each school is different. LSAC considers any jump of 11 points or less as typically non-notable. So, for me, that's the point I start to consider if an addendum is needed. And last, the reason for the jump is hugely important. It has to be more than "I studied more." That's just not a good enough reason (as I note above). However, if there is a solid reason (technical issues, sickness, bad news just before the test, etc), then I might go lower than that 11 point mark to write an addendum. The question is: does your explanation satisfy on all levels as a viable reason? If yes, then write away! If not, be careful; you may actually convey something negative without realizing it.
If something above doesn't make sense, just let me know. I'm happy to explore this in more detail