I think the problem is that OEFA is not a dimension in the bar chart. As far as I can see you have two records per day (one for AMED and one for BMED). To calculate the bar, you are dividing by distinct date, so for a week you would sum 14 records and diivide by 7, which results in 2x the correct amount which is what you are getting.
A quick workaround would be to update the expression for the chart to
=sum(CASES/OEFA_BETT) / count(distinct KL_DATUM) / count(distinct OEFA)
Hope that helps