Project BIlling - how does billable flag get determined for burden transactions

Here is our scenario - we changed some billable flags on timecards from billable to not billable.  Before running the burden on them, we changed them back to Billable.  However after running the burden, the burden transactions were still created as Not Billable.  I thought the burden transaction should follow the raw cost transaction as it stands when the burden is created. (We burden as separate burden items) Why does this happen - it has happened more than once to us.