AR Reference Accounts

When we want to add a Reference Accounts on a customer, for some Business Units, the receivable account is defaulted. For some however, the receivable account remains empty.

We have checked Manage AutoAccounting Rules, Manage Receivables System Options, Manage standard lookups and Manage common lookups, but were not able to find where the value for the receivable account was derived from.
In all these setting, there were no difference between the business units relating to this issue.

We did notice that our older Business units all seem to have the value defaulted, but Business units we have created more recently do not.