The optimization is designed to maximize your benefit in the long term. You may notice a small benefit if you override the optimization, but this override will claim donations that might bring a higher return in future years.
If you are getting a bigger refund when manually adjusting, then you are increasing donations claimed provincially, and wasting donations federally. Donations are a non refundable tax credit, which goes against your tax payable. The optimizer makes sure the donations are wipe out as much tax owing as possible, without letting the donations go to waste. Since federal tax and provincial tax are calculated differently, the donation amount used to wipe out your tax owing will be different.