A pivot table total is evaluated as expression total (like in a text box). It is not a sum of rows and you can't choose from the total mode options that are enabled in straight table only.
I assume your base billing values have a different / unlinked relation to your dimension values than your Actuals, so you get a total in all lines. If you want to use a sum-of-rows also for a pivot table total, check out the chapter in the Help, it's explaining of how you can do this using advanced aggregation.
It will look like:
= sum( aggr( sum( [Base Billing]), [End Date]) )
Hope this helps,