Audit Trail Column: AUDIT_TRUE_NULLS

Greetings,

I have just started a customization that requires the use of the Oracle Audit Trail Application.   I will be auditing the FND_FLEX_VALUES table for a specific value set.   I have created the audit, as per a couple of articles found on the web, and modified the created the procedures so that I am auditing a specific value set. All appears to be working well except the column AUDIT_TRUE_NULLS is null for most updates.   I was under the impression that this column contains a string of Y/N values, one for each column modified/updated.

I would like to understand why this column is null when an audited column is updated.  Is this normal behavior or a possible flaw in my audit setup?  The two rows below depict my confusion.  The row updated by Audit_User_NAME AAAAAAAAAA has the Audit_True_Nulls populated while the row updated by audit_user_name BBBBBBBBBB does not.  The data is from the _A table.