By far the best explanation of the LSAT conversion. I repeat, your score is not influenced by the other Dec takers. The LSAC (with their many years of expertise) create a test that should get a normal distribution of scores and that each of those scores can reasonably be compared to the same score of a different administration.
In other words, a person that scores a 170 on a "hard" test should also score a 170 on an "easy" test.
They use the experimentals to make this determination. They see how the 170's did on certain questions and how the 160's did on certain questions, etc. and then construct a test and a "curve" to get the numbers they are looking for.
This works because so many tens of thousands of people take the test that it all comes out in the wash. If only 20,000 LSAT idiots took the December test, the "curve" would still be the same (i.e. probably -11), but most of the takers would just get terrible scores. The test was fair, but the takers were lame.