If you add a category to budget when you have previously posted transactions with current category, the previously posted transactions will not become part of Project Budget Balances.
The transactions will show in Unbudgedet Costs until approval of category in Budget/Budget Revision, but they will be removed from Unbudgedet Costs upon approval of category.

When you are revising the budget and add new categories, you will not now see the correct status in Project Budget Balances, because the previous posted costs are not included. Since the previous posted transactions are removed from Unbudgedet Costs, then the transactions should become part of consumed costs in Project Budget Balances.