When you create an invoice with multiple items, the total tax liable is calculated on the final invoice amount and displayed at the bottom. The tax amount is split up with respect to the different tax groups the invoice items fall under, rather than the tax calculated on each item.
Consider this scenario.
Item A in your inventory is subject to a regional tax of 5%, while Item B has sales tax of 2% levied on it.
If your invoice consists of 4 pieces of A and 4 pieces of B,