It is not likely to be the judges' fault. The terms will be laid out in law and precedent, including the discount. It will also be partly about what charges the prosecution brought them up on. The judge can't (and shouldn't be able to) just say it doesn't feel long enough so they added 10 years. While it seems counterintuitive, judges should not have large amounts of individual judgement, and things like this should always be set out in a framework that all judges follow. Otherwise you get significant differences between judges (a book that talks about this is "Noise: A flaw in human judgement").

If we think that this should have a longer jail term, then that should be set out in law, not something an individual judge decides.