From a couple of articles, one from the Wall Street Journal and one at Law Week Online, you might get the impression that some of our federal judges are stepping outside the bounds when it comes to sentencing. Some knowledge of the history of the Sentencing Commission and the Guidelines is in order. As you can see from the links, there is far more than I can write about here.
The short story is that the sentencing guidelines came about to address the wide disparities seen in sentences for similar crimes. One reason these disparities occurred because a judge would hand down a sentence that involved a minimum to a maximum range of time. The precise amount amount of time would be determined by the US Parole Commission which is essentially defunct now except for those who committed offenses before Nov 1, 1987. Now there is no parole for federal convicts. You serve the time you're given, but it is possible to have it reduced by 15% for good behavior.
The sentencing guidelines were mandatory until four years ago at which time the Supreme Court declared that mandatory requirement unconstitutional and declared them to be advisory. So now judges have more leeway to act as judges and take into account all factors of the case before they hand down a sentence.
Something like 95% of all federal criminal cases never go to trial. They are settled via plea agreement. Oftentimes the plea agreement allows the defendant to plead guilty to a lesser offense or fewer offenses which, based on the sentencing guidelines, results in a more lenient sentence than what they would've received had they gone to trial and been found guilty of everything they were charged with. Plea agreements themselves are a double edged sword when it comes to justice being served and expediency/economics. The defendant can accept a deal that results in less prison time and US Attorney gets a conviction without the time and expense of a trial.
An important aspect of being a judge is the ability to act as a judge which I believe to be extremely difficult. Mandatory sentencing guidelines and mandatory minimum sentences make it easy--unless you find that this person and the public would not be best served by being incarcerated for that amount of time. Advisory guidelines allow judges to act on that, but I don't have room to get into all kinds of specifics here.
I invite you to contact the US District Court in the Eastern District of Washington and ask for the sentencing scenarios DVD. Created by Judge Whaley, it takes you through several scenarios where you get to be the judge. This is particularly appropriate for groups, such as students, where the groups listens to each scenario, each individual decides on the sentence, and then the group discusses what everyone came up with. You'd be surprised at the disparities as well as the agreement/disagreement with the advisory guidelines.
One side note. The combination of mandatory sentencing guidelines, mandatory minimum sentences, and the criminalization of more behaviors are factors that cause our country to have more people in prison than any other country.