The presidential veto on law ...

No it isn't a miscarriage of justice. The jury is used primarily to pass the judgment of guilt or innocence. Sentencing is a whole different story and sometimes the jury passes a sentence that is too harsh and outside of the guidelines for the crime. I know a guy who wanted a jury trial in an assault case. The maximum jail sentence he could have gotten was 2 years. A jury would have given him that after hearing how he beat the crap out of his girlfriend while she was going to the bathroom. But this guy didn't have that big of a prior record and 2 years would have been reduced by the judge because of that fact. The most he would have gotten would have been 3 months. There are plenty of factors that a judge has to consider that a jury doesn't always. I know this is completely off the scenario you gave but it is more realistic as to when this sort of thing happens.

I'm not going to say that it's necessarily a miscarriage of justice. Remember the judge knows the law much better than any juror would and that includes sentencing laws. Without knowing all the facts of the case I can't say one way or another if the judge was right or not.