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 (Average Daily Balances), 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 your connector 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.  (A pivot table is useful for getting the sums and then averaging.)