The deferential policy from a 2004 Yates Memo:
In matters relating to an extension of nonimmigrant petition validity involving the same parties (petitioner and beneficiary) and the same underlying facts, a prior determination by an adjudicator that the alien is eligible for the particular nonimmigrant classification sought should be given deference. A case where a prior approval need not be given deference includes where: (1) it is determined that there was a material error with regard to the previous petition approval; (2) a substantial change in circumstances has taken place; or (3) there is new material information that adversely impacts the petitioner’s or beneficiary’s eligilbity. Material error, changes circumstances, or new material information must be clearly articulated in the resulting request for evidence or decision denying the benefit sought, as appropriate.
Read the full USCIS memo in pdf format explaining deference for extensions (and exceptions).