It seems that adding a product to different categories on different rows actually forces the second category to become a child of the first category (parent/child).
I can see that happening if the category info is in CAT1_Name1 field and the other category names are left blank. Make sure that you start filling the category names from CAT5_Name1 field to CAT1_Name1 field. The data tool treats CAT5_Name1 as the root category so a row must always have data in this field.