as I explained before, that is the workflow that triggers the tax calculation.
when the user first adds an item to the cart, they have not set the billing state, so tax cannot be applied. You have no way to know if they are from California or Timbuktu.
On the checkout page, they set the billing state. The checkout page posts to the confirm page where the billing state is now stored in a session variable.
That session variable is used to determine if tax applies or not.
the scope of a session variable is to maintain it's value until the browse is closed.
when you go back to the cart page, the session variable still has the selected billing state, so tax can be triggered to display it.