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Sorry if this has been asked and can be a stupid question

Why is Prep 18(December 1992-that could have been Prep 7 as October 1992 is Prep 6) not following a chronological order?

Is this prep test different from others in terms of difficulty? Any idea?

The December 1992 test was unreleased (like the Feb tests today) but was later released as a PT by the LSAC in 1995. That's why it's out of order.