Spartan 6 pin output impedance
The datasheet values are across the recommended operating conditions (-40C to +100C Tj for industrial grade); IBIS models are usually at 25C and do not account for large variations in temperature.

This is a common issue in models, as the typical (not guaranteed) values are used.

If you expect to see a wide range of temperatures, use the values from the datasheet, as these are the only values that are guaranteed (if you have problems and used the IBIS values, Xilinx will not help you as those values are not guaranteed).

If you have a need for precise output terminations, then do not use the internal terminators.