Remember that Metformin reduces insulin resistance a little, this results in insulin levels that are a little lower, and therefore our body reducing its own insulin resistance that then results in insulin levels that are once again a little bit lower REPEAT..... Hence we need to consider the long-term effects of Metformin on our insulin resistance, not just what our BG says over one week.
The above does not work, if we eat up to our meters, and hence stop the cycle by increasing our carbs. We can clearly make the above cycle go a lot faster by "low carb" etc, and without diet changes, Metformin just slows down the increase in insulin resistance.
When in double-blind studies, Metformin people were given Metformin or a "fake pill", the people on Metformin had their long-term A1C reduced by about 10, some people had it reduced by as much as 20. Yet we all know that on a day to day bases, we can't see much improvement from Metformin on our BG meters, as unlike "low carb" the effect cannot be measured on a meal by meal bases.