Look. Obviously, some of the people who score 180 are getting lucky. Think about it this way: Assume that everyone who is a natural 175 scorer will be guessing on the remaining questions. Given that you have to answer five additional questions correctly, and the chances of getting a question right on a random guess are one in five, then one in 3,125 people who are natural 175 scorers will score a 180. (In reality, the odds are somewhat better, because most of these people will be able to eliminate at least one wrong answer on most questions. But the odds are still very much against this happening.) In fact, though, we know from published LSAT percentiles that more than 1 in 10 of the people who score 175 or higher actually get a 179 or higher. This is so much higher than random chance would predict that you have to assume there is usually some skill involved in achieving the higher score.
Secondly, I know of at least one person who took more than a dozen practice tests, and scored a 179 or 180 on 7 of her last 8, and then got one of those scores on the actual test. Either this person had a run of luck that statistically boggles the mind, or there are actual striations in skill level that are meaningful.
Obviously, any of these scores are tremendous accomplishments, but it seems reasonable to assume that a 180 scorer is highly likely to be a better LSAT taker than a 175 scorer.
Further, the test is just as precise in these ranges as it is in any other range. There is likely just as much difference between a 179 and a 175 as there is between a 169 and a 165.
I agree with you for the most part, Southside, but this last bit seems off to me. I think you have to look at differential item performance. Some questions are easier for the majority of testtakers than others; the hardest questions (and some random issues like misbubbling and fatigue) determine the striations at the top.
I think misbubbling and fatigue are basically constants that affect all levels equally. Why would a 155 scorer not be just as affected by misbubbling and fatigue as a a75 scorer?