When deferred is unchecked, the queue item will be committed at that step in your activity. When deferred is checked, the queue item will be committed when the next commit happens in your process. The deferred option can be useful when you want to make sure the queue item is not committed if there is another failure or if you want to make sure the queue item is not available until after your work object is committed.

What do you see when tracing your activity? Please include the DB Query event type so you can see the commits. Are you using the pyDuplicateProperty check in your queue item? When this is set, the code will check to see if a queue item already exists that meets the criteria. If there is already an existing queue item, then a new one will not be created. If this property is not being used, then the code will just do an insert without checking for existing entries.