I suspect that the GX550 will have cupholders in the cargo area, whether you buy a 2-row or a 3-row. Plastic molds are expensive. Having two molds, one with cupholders and one without, plus having to inventory two different parts, would be a needless expense.
I'm not quite sure why you would care, though.
You say that, but the GX460 has two different depth cupholders available for the third row, depending on whether or not you have the ML audio package.