Questions on the relationship between IBIS model & datasheet

Dear SI & IBIS gurus,

Hi! I'm a newbie in the world of SI and I have question that some of you
may be able to explain it to me.
I have a datasheet and an IBIS model of a same device. On the datasheet
(see pg 4), the Ioh and Iol values are -24mA and 24mA respectively when Vcc
is 3V.
Now, on the IBIS model, at the pullup and pulldown section, when the
voltage is at 3V, the current are not anything near to 24mA or -24mA at all.
My question is:
1) Why is that the datasheet values do not seem to correlate with the IBIS
model?
2) How do we know that the IBIS model is correctly generated based on the
values from the datasheet?

I welcome any comments or explanations on this issue. Thanks.

Regards,
Su Ming TAI