When the LLG created rafsi, they were assigned partly on the basis of how often the corresponding natural-language equivalents of the gismu were used in compounds. That is, gismu corresponding to concepts that are often used in compounds in natural languages, would get many rafsi, and gismu corresponding to concepts that almost always appear alone in natural languages, would get no rafsi.
However, it soon became apparent that Lojban usage did not follow natural language usage, so the rafsi assignments were not as useful as they were meant to be. So in 1993, the LLG generated a statistic of how often and in which positions the rafsi appeared in lujvo, and on that basis proceeded to assign new rafsi to the gismu, in order to make as many lujvo as possible as much shorter and less crunchy as possible.
A comparison between the old and the new rafsi list can be seen here: http://www.lojban.org/files/history/bothraf.txt
This has the side effect that Lojban written prior to 1993 looks funny to people who are reading it now. For example, xisli'icutci (rollerblade, xislu linji cutci) has survived in the lujvo list for many years, even though by today's rafsi assignments it means xriso linji cutci! The Lojban spoken between 1987 and 1993 is sometimes poetically referred to mijyjbo, "Middle Lojban", in reference to a poem by Michael Helsem (see LojbanicMythology).