Custom Sets causes weird behavior - looking for clarification

[EVilGnome]

In my package, I have Object Account and Sub Account. I have created a new data item, Obj Sub, that combines these fields. I have created custom sets using a set item with the expression "filter([Obj Sub], [Obj Sub] in ('51000.001', '51000.002'))." Then I have a data item that provides one total for the set, "total(currentMeasure within detail [Set1])."

I have created a crosstab with these totals as my rows. It does not, however, work in this raw form. I get the error: "Unable to build the aggregate dataItem= <Total Name>. The referenced dataItem = <Set Name> was not found at the same nesting level.

To fix this error, I create another row with my set item and the [Obj Sub] item. I hide these by using box type=none because I only want to see the summary information.

For my understanding: Can anyone explain what is going on here and why the difference? If I create the sets using the intrinsic property [Object Account], I don't need bring over the set items. Why is this only when I create the data item?

Further, this solution works for an aggregate total, but % of total does not work because of the rollup aggregation. Any ideas on how to resolve this issue?