Reports in Community analytics is driven by the event logs present in the time frame selected for creation of the report. If any member was not active within that time period, no event pertaining to that member will be present in the bulk member data report. Hence the mismatch between the report from admin metrics and CA.
So, there's no foolproof way to retrieve all member data in CA as you will always have a sub set of members who may not even have kudoed/viewed/commented in that time period.
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