Because it's joint household income for taxes. The I-864a ensures the spouse is fully aware of the obligations of sponsorship. Some consulate officers require it.
Since your parents are not listed as dependents on your tax return, I assume they are not financially dependent on you. Therefore, I would not count them in the household size.
It's an update to and a continuation of your previous case history, thus I merged them. Merging the threads allows for continuity between your posts since it is a continuation of the same case.