When federal sentencing was reshaped with the idea that it would be fairer for everyone if sentences were similar across the country, these sentencing guidelines were actually requirements. All sentences had to fall within the range of possible sentences offered for a certain offense. This meant that even if the sentence seemed too harsh for the specific case a judge would have to uphold it.
After the United States v. Booker Case in 2005, the federal sentencing guidelines took on an advisory role. Judges are still required to consider the guidelines and make a calculation based on the facts of the case, but they are able to change the sentence to make sure that the punishment is one that is right. In some cases, this may be something less than the one in the guidelines.