The bus skew reporting for the OFFSET OUT constraint in the presence of the REFERENCE_PIN keyword is incorrect.
This problem has been fixed in the latest 10.1 Service Pack available at:
http://www.xilinx.com/support/download/index.htm
The first service pack containing the fix is 10.1 Service Pack 2.
NOTE: Remember to place the reference pin into the time group of pads for the OFFSET constraint. If the reference pin is not in the time group of pads for the OFFSET constraint, the OFFSET table reports incorrect skew values.