What value are you expecting when you remove PO from drill-down.Let me clear my point with simple illustration.
| PO | Purch.org. | Start date | End date | Difference |
|---|---|---|---|---|
| 10001001 | 1000 | 20.02.2014 | 22.02.2014 | 2 |
| 10001002 | 1000 | 20.02.2014 | 24.02.2014 | 4 |
| 10001003 | 1000 | 22.02.2014 | 28.02.2014 | 6 |
| 10001004 | 1000 | 24.02.2014 | 30.02.2014 | 6 |
So after removing the PO you will get only one row.Correct!
Now in difference column what value you think should appear..last value i..e 6 or the sum of difference i.e 18.
Note:It is mandatory to keep the reference characteristic in rows in case of you are replacing with the key of infoobject.This may also apply for attribute value as well. so that replacement path works properly.
Please check this note as well
1385580 How does a formula variable with a replacement path work?
Hope this gives an idea.
Regards,
AL