It may be because:
1.You chose option B which uses the budget assigned to Adext AI, plus, the budget of your original ads. Option B doesn’t stop running your original ads and because of this reason the budget increases its amount.
2. Because sometimes Adext AI can show a delay while reading data, since Google and Facebook don’t show spending in real time. This is why, our AI may read that there is still more budget left than what there actually is and may spend more than what was originally considered. This is because our AI needs to reassign the budget once again among the produced clones.
Note: Adext AI can only spend the budget of one day. If it spends more than that it may mean, there’s a bug.