Studies consistently show that individuals with higher initial HbA1c levels tend to experience more significant reductions when starting metformin therapy.

This relationship exists because patients with higher HbA1c have a larger "glycemic burden," which gives more room for improvement. Metformin, by reducing liver glucose production and increasing insulin sensitivity, can have a more dramatic impact in those with poorly controlled blood glucose. Research indicates that for every 1% increase in baseline HbA1c, the reduction in HbA1c tends to be more significant, particularly in those starting with HbA1c levels above 7%​(Article Change in HbA1c Across the Baseline HbA1c Range in Type 2 Di...

https://diabetesjournals.org/care/article/29/9/2137/24147/Lower-Baseline-Glycemia-Reduces-Apparent-Oral

However, as patients approach lower HbA1c levels (closer to normal), the reductions become smaller, which explains why patients with lower baseline HbA1c may not experience as large a decrease​ Article Quantitative Models for Evaluating the Correlation between B...

Article The mechanisms of action of metformin

https://www.nature.com/articles/nrendo.2013.256

Article Metformin Use in Adolescents: Old and New Therapeutic Perspectives