Why is there a billing rate on the time record when there seems to be no billing role on the assignment?

If a billing role was previously selected for a task assignment, the associated billing rate is applied to the time entries that were submitted during that period.

To view the billing role history, go to the edit task view and select the assignment. In the Assignment section,  click Actions > Change Billing Role and a dialog will show the billing roles previously assigned to the assignment, and the date of change.