Question : vmware vsphere 4.0

Hi Experts,
I have running vsphere4.0 with 3 hosts and 10 vm's.
I have seen something strange on my datastore.
4 datastores I have and the vcenter is on two.
When I click on datastore ms1 I see the vcenter with 32 GB used space.
When I click the datastore ms2 I see again the vcenter with 0 GB used space.

How can it be ?

Answer : vmware vsphere 4.0

At some point, was the VM migrated from one datastore to another?  The VM has 0 bytes of data on one datastore.  Does it have an ISO mounted on that datastore?  (This would also cause it to list that datastore in use.)