Category Budgets vs Spending Budgets

How come it is possible to set your spending budget lower than the sum total of your individual spending category budgets ? At the very least it would be nice to see the total of the category budgets in the app.

Also, I thought if you made a payment into your account and tagged it accordingly it would credit into the relevant category budget - that seems to be the behaviour for most, but for some reason isn’t the same for my expenses budget … my expenses were paid, I tagged it as such, so I would expect to see that credit back into the expenses category … or is my understanding of how it works incorrect ?