How is there a widespread differential in sentencing outcomes based on a judge's leniency? When both sides present their recommendations, isn't the judge required to formulate a sentence informed by those recommendations/joint submissions with regard to the mitigating/aggravating factors that are fairly objective?

I've never bought into the idea that a lenient or overly-draconian sentence not informed by standard practice and sentencing principles wouldn't just get struck down or severely modified on appeal