Exactly. The rotors being thinner than spec probably isn't the issue, but if the rotors are worn out they could have other problems with surfacing and contamination that could be causing at least some of the issue. If they've been overheated, glazed, contaminated etc it can cause issues. And if they were really overheated at some point it could even have changed the heat treat on the metal which can change the wear characteristics. Usually they have to glow red for that to happen so it isn't super common, but it does happen. People do dumb stuff, so if you don't know the full history of the rotors almost anything is possible.