In a recent meeting, Nebraska Service Center has clarified why, some times, in cases of pending I-485 applications, USCIS approves AOS interchanging the applications of the derivative and principal applicant:
485s can be amended if there is alternate eligibility. The priority dates assigned to one
individual can be moved between classifications within the parameters of the Pearson Memo.
If current principal husband has an approved I-140 (EB-2) with a more recent priority date and
the derivative wife has an approved I-140 (EB-3) with and earlier priority date, you cannot
switch priority dates and assign the earlier date to the husband. However, if the EB-3 for the
derivative wife is current, the husband's I-485 can be re-classified as the derivative and the
wife's as principal, to permit the approval of the applications.