Inventory transaction is in inventory but is not reflected in install base

We have a scenario where an inventory transaction is not reflected in install base for a non serialized install base trackable item.  So at this point the inventory on-hand is 3 and the install base quantity is 2.  We know if we run the Oracle Install Base Error Correction and Synchronization with mode of execution of S the quantities will be synced.  Is there a way though to find out why the transaction reflected in inventory is not in install base?  There was not a SFM issue of note that day.  Any suggestions would be greatly appreciated.  Thanks.- Stacey