Officers Sustained Form and the Aaronic Priesthood Presidencies

I was looking at the Officers Sustained Form available online, and I noticed that the presidencies of the Aaronic Priesthood (deacons quorum, teachers quorum, and priests quorum presidencies) are listed as being sustained by the general ward body.

Handbook 2 says that these officers are sustained by quorum members, and the Young Women presidencies and Melchizedek Priesthood presidencies/group leaders are sustained in their individual classes, quorums, or groups. Why are the Aaronic Priesthood presidencies sustained by the general ward membership instead of in their individual quorums?

Thanks,
David Smith

The "Officers Sustained Form" is used during Ward or Stake Conference. In that, we sustain a number of positions that are not sustained by the general membership at the time they are called. For example, the Prophet, Quorum of the Twelve, area leaders, etc.