Your asset data is shared by your custodian with BillFin. They do not send values on the weekends or on holidays which means that they provide positions for around 65 days a quarter. BillFin takes the account's position totals for each day and averages them over the number of days for which your custodian provided data. You can actually confirm the calculation.
If you are set to bill on Average Daily values, after billing, go to the pencil menu for any client and click on View Asset Values. Then click on View in Excel. Open the spreadsheet and you can see all the positions provided by TD for that client's accounts. If you sum the positions for each account for each day, and then average those sums, you should match BillFin's "Billable Assets" value.
I set the pivot table up as shown in the image below. I don't use the pivot table's automatic calculations because they continue averaging at the total levels. I go to the first empty row below the totals and I sum values for each day and then I average those values. Let me know if you need more details.