I have seen this happening in our environment and one of the reason I could find is that the Removed field is set to true when the a specific probe like the vmware probe monitoring this VM reports that the VM is removed from the configuration. It could be that the VM was being associated with one ESX server and later moved to another ESX server or the VM was removed from the vmware probe configuration.
When the above happens it doesn't automatically remove the VM but it just updates the Removed field to Yes. Multiple entries might be created if the reconciliation between the entries created by vmware probe and the robot does not happen properly
This is what I observed and there could be other reasons too, so I will leave this for someone from CA to comment on