When using the 9500xl IBIS model in Mentor Graphics ICX, I find that the IBIS parser fails due to non-monotonic behavior at various voltages.
The behavior you are seeing with the 9500xl IBIS model is because 9500xl uses dual power supply. There is 2.5V for the control circuit and 3.3V for buffers. The ibis model is based on HSpice simulation results. The same netlist was used to generate 9500 ibis model. The model does show the monotonic GND_clamp data since it used one power supply, 3.3V, for both control and buffer. Hspice simulation results show that the non-monotonic ground clamp characteristic is the part of the circuit behavior of dual power supply. It should not be treated as an ibis error. The ibis checker realizes this and gives the following warning messages:
WARNING (line 236) - GND Clamp Typical data is non-monotonic
WARNING (line 236) - GND Clamp Minimum data is non-monotonic
WARNING (line 237) - GND Clamp Maximum data is non-monotonic
Errors : 0
There is a switch in the ICX tool that will deactivate the IBIS model verification in ICX. Please contact Mentor Graphics for more details.