Or, and this may be completely out of this world, he's heard the evidence, read the reports he's legally obliged to read, and followed the laws and guidance that he legally has to and worked from that?
As opposed to some headline from a paper where they've rarely if ever actually sent anyone to watch the trial and want headlines and outrage.
Judges work to the wording of the law, past precedent, and the sentencing guidelines as laid down by the government, they basically cannot go against them, they also take into account things like the government telling them "we've only got a couple of dozen spare prison cells in the country, please only send the absolute worst to prison" (IIRC our current government has been reducing the number of prison places and they've mucked up a few times resulting in very low hundreds of spare places in jails at times).
Anyone can appeal a sentence that they feel is too lenient via a relatively simple process, at which point IIRC if the appeal is felt to have any merit another judge looks at the case including the judges reasoning, which is often a fairly lengthy report showing their workings in the same way you had to show your workings for maths homework with the things they've considered in their sentencing and how they feel they should be applied and the legal reasoning behind those consideration.
Judges, despite the way some idiots think, don't just pull a sentence length out of their backside*, they have to work within some quite limited constraints so that if there is any appeal or anything other judges can see exactly why they did what they did.
*Unlike in the US where a lot of the lower end judges are less qualified and less supervised than our magistrates (who are actually trained to some extent and have to listen to a highly qualified lawyer who advises them on the law).
