Unfortunately you have not said which data you actually archive (SD, FI or general data).
And you also missed to tell what settings you made in the selection screen.
In case you archived SD data then it might be that this customer is used as a partner in other customer masters. Check in table KNVP.
If you archived General data then it might be that this customer is linked to a vendor master e.g. for subcontracting order with delivery of components. Check LFA1 KUNNR field
In case of Fiance data there might be link as head office or alternative payer in the KNB1 table.
You can omit the checks from the selection screen (not really recommended)