Primarily because a local report can only take into account what it knows. The Area report must take into account the information from ALL the facilities in the Area. To the extent that a population is seen at multiple facilities, potentially reporting different information to each facility, you can expect the Area report to vary from the local.
Secondarily, even in the case where a person is seen only at
one facility, the local report is created by another office and may not use exactly the same reporting criteria (i.e., most local reports use beneficiary code ‘01’ to determine whether someone is an Indian, whereas the NDW uses an algorithm based more on tribe. Refer the the section “Deciding who is an Indian”.