Hi Mike,
The basis of this behavior is that Storage DRS only considers the 1TB provisioned space instead of the 64GB. By nature Storage DRS is very conservative, as it cannot determine the growth rate of the data inside the VMDK. Therefore it tries to avoid a DOS for other virtual machines and assumes the VMDK is going to be 100% utilized.
I wrote about this behavior in the whitepaper "Understanding VMware vSphere 5.1 Storage DRS" http://www.vmware.com/files/pdf/vmw-vsphr-5-1-stor-drs-uslet-101-web.pdf
This behavior applies to Storage DRS in 5.0 as well.
I expect the datastores in your datastore cluster do not have enough free space to host that "1TB" VMDK, therefor Storage DRS is "defragmenting" its datastores. For more info on this process please review this article: Storage DRS initial placement and datastore cluster defragmentation - frankdenneman.nl
Hope this explains your scenario
Frank Denneman