Alberto Ferrari explains a nuance of summation in DAX:

In simple DAX measures, the total of a report is the sum of its individual rows. For more sophisticated measures, the total might seem wrong because the formula does not include an aggregation over the rows that are visible in the report. For example, if the total of a measure must be the sum of the values displayed in the rows of a report, we consider the expected result a “visual total”, which is a total that corresponds to the visual aggregation of its values sliced by different rows in the report.

Click through for a straightforward demonstration.