From the horse’s mouth (via @christine ‘s article on the homepage here):
What is the “ethics adjustment”?
The Board of Governors instituted a policy to place particular
emphasis on ethics. Starting with the 1996 exams, the performance on the
ethics section became a factor in the pass/fail decision for candidates
whose total scores bordered the minimum passing score. The ethics
adjustment can have a positive or negative impact on these candidates’
CFA Institute has a policy of not releasing either the minimum
passing score or individual candidate scores. Consequently, CFA
Institute does not release specific information about the ethics
adjustment or the candidates who were affected. The adjustment has had a
net positive effect on candidate scores (and thus pass rates) in most
exam sessions. The published pass rates always take into account the
ethics adjustment for borderline candidates.