If the Destination Type is Expense, then the Charge Account will be defaulted from the HRMS tables

Please correct me if I am wrong for the below statement.

"If the Destination Type is Expense, then the Charge Account will be defaulted from the HRMS tables"

We found an instance where the Destination Type is Expense, however the charge account was left blank as the default.  

Do you know why this happened and when does the charge account get populated with the default from the HRMS table and when does it not?

Thanks,