How to customize Expense Report to display custom field: Sub-Account from Expense Category?

Hi

the expense report displays the expense account number when Category is entered. wnat to understand how it is happening. apparently it is picking the expense account from the Category defined on the setup->accounting>expense categories

we have included a custom field "sub account" on the expense categories form and Enter Expense reports form, now the "sub account is not defaulting on the expense report form similar defaulting is happening for expense account but custom field on expense report "sub Account" is not defaulting. do we need to do something else? refer the screen shots below