Inactive Customer showing in Collections LOVs but not Billing

Summary

Duplicate customers, 1 Active, 1 Inactive. Inactive shows in Collections but not in Billing.

Content

Client is interested in why an Inactive Customer number would be showing in the Collections List of Values when it is not showing in the Billing List of Values.  CustomerA has Registry 123 Inactive, and 456 Active.  In Billing searching on CustomerA brings back 456 only (as a side note, searching in Billing on 123 specifically does bring up the record).  In Collections searching on CustomerA brings back both 123 and 456.  Client would like 123 to not show in Collections.