I'm answering my own question:
The discrepancy is due to section access. I assumed that, because I'm admin with access to all, I would get to see ALL data in the sheets. Well, no.
My section access is based on a field called ORGID. As admin, I can see all data related to every value of ORGID. The detailed data reside mainly in two tables:
EHRdata (fields: ORGID, PatientID)
CLAIMSdata (fields: PatientID, Recipient_Unique_ID)
Therefore, any rows that I loaded into CLAIMSdata with a value in PatientID that doesn't have a match in EHRdata will NOT be visible in the app, because those rows don't have corresponding ORGID values, which are necessary for section access to grant me visibility into them.
Thanks for the opportunity to work this out for myself.