The SWMM 5 text output file has a Storage Volume Summary Table. The Maximum Outflow from a storage node includes ALL of the outflow from the Storage node including both downstream link flow and reverse flow into the upstream links of the storage node. You need to look at all of the flow out of a Storage node to find out how the maximum flow was computed during the simulation.
|Figure 1. Storage Volume Summary Table
|Figure 2: Total Flow Out of A Storage Node includes Reverse Flow.