Immigration Forum / USA Marriage Base / April 2006

Hi,
I have been researching this question for a while now, including
reading a lot of postings in this group, and I just don't get it.
Maybe the great minds of this group would be willing to explain this to
me?
Here it is:
I am a US citizen. I am about to file concurrent I-130/1-485 for my
wife and her 11 y.o. son from a previous marriage, both are on H visas
in the US. A lot of posts from experts here and also 2 customer reps on
the phone to CIS National Service Center tell me that I should file 2
separate I-130 petitions for my wife and for her son (who is now my
stepson by CIS definition).  But why?  In I-130 instructions in "For
Whom May You File?" it says :
1. Your husband, wife or unmarried child under 21 years old;
2. Your parent if you are at least 21 years old;
3. Your unmarried son or daughter over 21 years old;
4. Your married son or daughter of any age;
5. Your brother or sister if you are at least 21 years old.

"Stepson" or "stepdaughter" is not listed.

So I understand that I only file for my wife, and list my stepson in
her dependants.

At the same time they concurrently file two I-485 (one filled out by my
wife where in Part2 "I am applying for an adjustment to permanent
resident status because:" she checks "a" (an immigrant petition giving
me an immediately available immigrant visa number has been
approved...blah blah), and the other I-485 filled out by my stepson
where in Part2 he checks "b" (my spouse or parent applied for
adjustment of status or was granted lawful permanent residence in an
immigrant visa category that allows derivative status for spouses and
children).

So, I see the logic this way:
1. I petition for my wife to be eligible to adjust status (I-130)
2. When she applies for AOS, her son is eligible for AOS too, because
he is her minor child.

If I am wrong, could you please explain to me where I am wrong, and how
can I file I-130 if the forms says I can't file it for stepchild?