Please refer the SAP Note - 403254. It explains how the rounding difference happens and how the differential amount is handled....in the case of group condition.
The rounding happens at line item level. Similarly, the condition amount will be calculated at header level (@ 5%) also and after rounding it...system compares this header value with the cumulation of values from all the items. If there is a difference (positive or negative) between these two values, then the difference will be added to the line item (of that group) with highest net value.