Looks like its a known issue with respect to Tivoli. see the Troubleshooting section in http://pic.dhe.ibm.com/infocenter/tsminfo/v6r4/index.jsp?topic=%2Fcom.ibm.itsm.ve.doc%2Fr_ve_gui_troubleshoot.html.
Consolidating VM backups
After a VM backup, the VM might contain preexisting snapshots even though no snapshots are present in the Snapshot Manager. For example, the VM hard disk points to snapshot VMDK files (for example *-000001.vmdk) instead of regular VMDK files. Although preexisting snapshots might be intentionally retained, Data Protection for VMwaredoes not provide a mechanism to verify whether the VMDK points to a valid snapshot. When snapshots are not consolidated, and a VM with preexisting snapshot files is backed up, Data Protection for VMware might report an incorrect size for the backup on the Tivoli Storage Manager server. Snapshot consolidation also prevents other VMware related issues. As a result, consolidate your snapshots whenever this situation occurs.
To resolve this potential problem, VMware vSphere Client 5.x (or later) notifies you when a VM requires snapshot consolidation. Details about the consolidate option are provided in the following VMware KB article:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2003638