How to track Stock Difference?

My retail client is using external system to create delivery orders. Based on that delivery orders PO are automatically created in SAP and relevent transfer posting take place. In short external system tells SAP what to do.

Now every week a report is generated which tells the SAP stock and External Syatem stock. For many articles(Materials) there is stock difference. Now I need to know why this diffence is coming.

Is there any T.Code or any report which can tell me for a particular article(material) what posting has taken place and how stock is affected by it in specified interval of time?