There are some orders which belong to reseller's customers. A tax was not applied to them.
A tax is not configured for a reseller. Provider may see this the following way:
Taxation Manager" for a reseller through
PCC > Top > Configuration Director > Security Manager > Setup > Reseller Permission > Reseller Administrator: Billing Director > Taxation Manager
Login to Reseller Panel (RCC) and see that tax zone is not configured:
RCC > Login to RCC > Top > Billing Director > Taxation Manager > Tax Zones
That explains the reason why tax was not applied.
It’s by design that provider's tax is not applied to orders of reseller's customers.