I found following in the forum, and as expected, the evaluation is different. It would be a help to see the data used for the example giving the "wrong" KPI
Auto means calculate the total by evaluating the expression over all data. Sum means to sum up the rows in the chart. There can be differences depending on your data model and dimensions used. For example, Auto will never double count. Sum can double count when for example the same fact is connected to more than one dimension in the chart.