Why is the hydrograph window Maximum Discharge value different than
the DesignQ value for some entities?

The Maximum Discharge Q value for an entity, is actually the peak value of the hydrograph entering the entity.  The DesignQ value for an entity, is the peak value of the hydrograph within the entity.  These values are usually the same, however, in some cases, they can be slightly different.

Pumps, diversion, and pipes receiving large amounts of defects flow are examples of situations where the values may vary.  The Maximum Discharge Q is not available for output in reports.