Hi - metformin doesn't actually reduce blood glucose directly. What it does do is interfere with the liver's ability to top up your BG when the liver thinks it needs it - and that can be for many reasons. I need to say I've never taken metformin myself so have zero personal experience. My preference is not to take medication that I don't need to take.
Info from Bilous and Donnelly, "Handbook of Diabetes" -
Metformin increases insulin action (the precise mechanism is unclear) lowering glucose mainly by decreasing hepatic glucose output.
So how metformin works isn't fully understood, according to the text book. It might reduce HbA1c overall by a few points, and that effect is going to (probably) be different for different people.