Thinly provisioned virtual disk doesn't reuse empty space

I have a thinly provisioned 100GB data disk created with RHEVM 3.0.  I can see the allocated storage grow as I create a 50GB file.  After deleting the 50GB file and confirming it's gone with a "df -k" on the VM, I create a new 50GB file.  Rather than re-using the previously allocated 50GB, another 47GB is allocated for a total of 97GB allocated even though I never actually had more than 50GB on the virtual disk.

I'm using FC SAN.