Duplicate rows in subgroups

Hi,

I have a report where I created the Category group, inside it the Subcategory group, and inside that the KPI group. It looks fine in the Design pane, but in Preview there are extra rows with data for each of the groups: 3 extra rows for Category, 2 for Subcategory and 1 for KPI, and they are equal to the value in the previous parent row. Why do they appear and how can I delete them?

This is how it looks in the Design pane:

This is the Preview pane (I highlighted the extra rows):

Thank you,

Adina