I-864 using applicant income

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My wife and I were recently married and we are working on her AOS. I am a U.S. citizen by birth, and she is currently attending graduate school inside the United States with several more years to go. Her visa status before we were married was F-1 single entry (Iran). She spoke to an attorney who was fairly adamant that her graduate student income could not be used to meet the I-864 financial threshold, but as I'm looking at the I-864 instructions I'm having difficulty understanding why that is the case and was hoping that someone could explain it to me.

More background (if it helps):

I am currently a full-time undergraduate student and my own income does not meet the 125% threshold.

Our incomes together (my wife and I) do meet the threshold, fairly easily it seems.

The two of us filed our most recent tax returns as "married filing jointly" - her source of income (part of it) was taxed.

At the moment we do NOT maintain the same residence. We go to school in different states; we are living together for the summer and I will be moving with her December 2011 when I complete my program.

My wife can provide evidence that her source of income (a graduate program in the U.S.) will continue so long as she remains in good standing.

Obviously the two of us are going to speak with the attorney again, and my family is willing to sponsor her, but we would prefer to do this ourselves; any ideas as to why the attorney told my wife that her income will not count?