It has a purpose, but the writer needs to be clear which groups are being compared. Saying that 80% of group's A value X fell under the median of group's B val

It has a purpose, but the writer needs to be clear which groups are being compared. Saying that 80% of group's A value X fell under the median of group's B value of X can be a useful number for some analysis, even when group B is a subset of group A. The author must be clear which group they are using where, as any confusion between group A and group B should be considered an error in the analysis.