What this stage is
Cassation is not a rehearing. By this point the verdict has entered into legal force and the evidence is fixed; the court is looking only for breaches of law significant enough to have affected the outcome. Arguments that the wrong witness was believed do not survive here. Arguments that the act was placed under the wrong article do.
Why classification carried the case
Article 228.1 is divided into parts, and the parts are separated by years. Part 4 carries ten to twenty years; the parts below it start far lower. Where a court has attached a part of the article that the established facts do not support, the sentence is unlawful even if every finding of fact is left untouched — which is precisely the kind of error cassation exists to correct.
Why the Supreme Court's own positions mattered
Cassation complaints that work are written in the court's own language: its Plenum resolutions and its published reviews of judicial practice. A complaint arguing that a sentence is unfair in the abstract is refused. A complaint showing that the classification departs from how the Supreme Court has said the article must be read is a different document, and this was one.