As well as the physical value, this report will also take into account orders pending in the Dimensions/Delta process. If an order is stuck, this may create a false positive in the report. Please check your Delta import batch, and ensure everything is up to date.
If your report is still showing a different physical value compared to what shows in Dimensions, Support will need to investigate this further and possibly run some scripts on the database. Any Issues with contacting us, please see the following Get support in Dimensions.