There can be a fine line between evaluating facts and making up new ones.
In Carroll v R  HCA 13 (21 April 2009) the CCA NSW majority had held that a sentence imposed for manslaughter was "manifestly inadequate". That ground for allowing the prosecutor's appeal amounted to an assertion that the sentence was unreasonable or plainly unjust. It is one of the recognised ways of challenging the exercise of a discretion.
Sentencing remarks have to be carefully cast, lest they stray beyond the offending that has been proved or acknowledged by plea.
Here the CCA majority had latched on to an objective aspect of the offending that was never in dispute - that the offender's violence was not justified - and used that to ignore a matter that was relevant: that there had been some provocation from the victim.
The second error by the CCA majority was to exaggerate what had been admitted. The unlawful act necessary for manslaughter – here a head-butt to the victim's face – was acknowledged by the guilty plea to be an unlawful and dangerous act that carried with it an appreciable risk of serious injury. But the CCA majority stretched this to "severe injury was clearly foreseeable and death at least a possibility", which was not acknowledged. All the offender had admitted by pleading guilty was that his act had carried an appreciable risk of serious injury to the victim.
Back then, goes this case, to the CCA for proper consideration of the prosecutor's appeal.