Jaren te veel huur betalen door fout energielabel

We are in the city of Groningen, where some tenants have been paying too much for years because the energy label on their home is incorrect. Who is responsible for the incorrect energy labels on hundreds of homes? Is there intent at play? And why is it so difficult for (former) tenants to get their money back, even if it is clear that something is wrong? We go to the city looking for answers.