I presume the reason could be even compiler being not able to quantify numerically the required resource utilization beyond what the device has built in, or also could be just a warning description not rightful.
That device utilization summary looks like the one produce in the ISE GUI after it goes through MAP. Have you looked through the XST report file? It will probably show that XST estimated more resources for the Slices than there are in the part. The report is <design_name>.syr I think.