Trouble expanding VMware storage on VNXe3150

I am trying to expand a datastore LUN on a VNXe3150 by 500GB.  I have deleted several LUNs to make sure there is enough space available.  When I look at the storage pool it says that I have 793GB available.  But, when I try to increase the datastore capacity it tells me the maximum size is only 484GB more.  If I try to increase it by 500GB anyway, it gives me an error: "There is not enough space in storage pool (Error Code:0x6401092)"  Why am I limited to 484GB for the datastore when the storage pool clearly shows 793GB available?

Rob