In 1984 Congress passed an act that made it so there were specific guidelines for how a person could be sentenced for a crime. Prior to this point in time, there was a lot of concern that judges in different parts of the country were being too lenient or too strict when it came to sentencing. Congress decided to make a set manual that could be used when sentencing people.
Federal sentencing guidelines suggest a sentence based on the type of offense, the history of a defendant, and how the crime was committed. These are still only guidelines of course. Judges may still decide to give a more lenient sentence based on factors that are presented to them. This decision must be seen as reasonable in response to the individual factors of a case.