This is because they are their in the page,shard file.
The reason for being them their is, the files are deleted before deleting the task associated with them.
Try below steps to remove them.
Create a Qvw file with the same name which you want to remove from the document tab.
Go to Document tab and delete the task and then restart the server, again go to the document tab and check the report name, they will be gone.
then you can delete those QVW files from the folder.
Surendra - I did experience this issue in User Documents. These may be old qvws residing in folders other than mounting folder. I searched through my server and cleared them up. Did you search all folders in your environment?
Koushik solution would work for the Source Documents. However, you need to search your server through Windows search and identify these files. I found it in QVSSettings folder and don't know how qvw was dropped there? It is locked down by the QlikView service account and deleted it manually logging in as Admin.