What changed between the earlier stages and this one
The courts below had assessed part of the evidence formally and had not given proper consideration to the mitigating circumstances. Article 307 of the Criminal Procedure Code requires a verdict to state the reasons for the sentence imposed; Article 61 of the Criminal Code lists mitigating circumstances the court must address. A verdict that lists them and does not explain their effect has a defect on its face.
Why this is a cassation argument and not an appeal one
Asking a court to be more lenient is an appeal argument, and it dies when the verdict enters into force. Showing that the sentencing reasoning does not meet the requirements the Code imposes is a question of law, and it survives into cassation. The distinction is the reason this complaint succeeded where a differently framed one would have been refused without a hearing.