Do you know something we don't? As far as I know, the automatic admit and deny cutoffs are kept strictly confidential. So, by viewing your index against a possible Max. Index or 75% index, an applicant can get an idea of their chances, but they can never know for sure.
Perhaps they do not publish their indexes because they do not want other top schools to know how their admissions procedures are tailored to help them move up in the rankings. In any case, they are using some sort of GPA/LSAT index, even if they do not use a fixed numerical one.