rising and falling time in IBIS and XTK

  Hello:
I have a question regarding the rising and falling time, I understand
that the ibis2xtk is going to translate the ibis model to Quad (xtk)
model, but when I compare the original rising and falling waveform (ibis
model) with the rising and falling waveform after the translation( Quad
model), I have found a big difference in the rising and falling time
(150 ps) between those two models.
I know that the translator is going to remove the non-monotonic points
inside the ibis model , in addition to that is there any more reasons
why so large in the time difference.
Is there any remedy to correct the rise time and make it closer to the
values with the ibis model?

I appreciate any feedback I can get in this regard.

Thanks

Jason Leung