As well as the physical value, this report will also take into account orders pending in the Dimensions and Delta process. If an order is stuck, this can create a false positive in the report. 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, our support team will investigate and run scripts on the database if needed. Any issues with contacting us, see the following Get support in Dimensions.