The VMware host would always report the correct space for the filesystem (in this case vmfs). You are experience the effects of the "high water mark". Common with thin provisioned systems. In windows this is handled with the Compellent Server Agent, in VMware its addressed with VAAI primatives.  As to the proper configuration of VMFS for Compellent - I would recommend reading the best practices guide. en.community.dell.com/.../20437942.aspx
I am about to call Co-pilot. But want to post here as well to see if anyone has encountered.  So for example you have a 1TB Compellent LUN. You create a 500GB vm on this LUN. You are now using 500GB of the Compellent LUN, and VMware will show you are using 500GB of the 1TB lun. Now you delete or storage vmotion the VM. Vmware now says this LUN is back to 1TB free. Compellent however still shows only 500GB free, due the blocks already being allocated I guess. We are on esx 5.1, but not on the update that solves some the of  the unmap issues which you can use.  But do you "really" need to use it? With Compellent showing 500GB free now, and VMware showing 1TB free, could I create a 750GB vm on this LUN?? I know I tested this many years ago, and I thought the answer was yes I could. VMware knows where to write, etc. I am leary of even using unmap even when I get a good bulid to use.  Also, we are using thick provision VMs. Thanks for any information. 
White space and VMware