Total Vat amount in Billing summary page

Hi,

While performing some activities in the billing invoice, we have come accross a scenario with the total vat amount which looks little confuse. In the invoice summary page, there is a field called Total Vat amount which we thought it will be generated by the billing interface vat default process for those transaction where the vat is applicable. But this either populated when we click the button 'Caculate Taxes' or by the BIIVC000 when the pro-forma or single action invoice job it run.

Can any please advise if this is how it should works as design or is there any set up that need to be changed so that this values can be populated from the interface process. Is there any reported issue related to this scenario.