When the United States Sentencing Guidelines were drafted, the idea was to create a fair system that was applied more or less uniformly across the country. Someone sentenced in one location should receive the same sentence as if he were sentenced in another federal district on the same set of facts and factors.
Of course, that goal has always proved somewhat illusory. Differences remain between, say, the Eastern District of North Carolina and, say, the Eastern District of Massachusetts. Indeed, differences in sentencing exist between judges within the same jurisdiction, with certain judges having a reputation for leniency or harshness.
(It should be noted that leniency is relative to the potential sentences, which are almost always uniformly harsh in the federal system.)
The chief way that the USSG have imposed this partial fairness is by creating a grid system where someone's prior record level (their conviction history) is cross referenced with various sentencing levels. The sentencing levels are determined first by the base level of the crime or crimes, and then the addition or subtraction of points based on various attributes associated with the crime or the person's conduct during or after the crime. For instance, early acceptance of responsibility might result in a 2 or 3 point reduction. Or the use of a gun - possession, brandishing, or discharging - might add various points. And on and on.
If you played the board game Dungeons and Dragons as a kid, think of it as kind of like that - with points creating an ultimate sentencing range from which the judge can determine a sentence. I say that judges can determine a sentence because under U.S. v. Booker the sentencing guidelines are merely advisory.
But through a combination of very punitive approaches to criminal justice, and of rules peculiar to the federal system, the main way that fairness has been imposed is by harshness. By this I mean, in the past deviations that seemed unfair, were usually leniencies granted in certain jurisdictions. Today, the system has been made "fair" by being made uniformly harsh.
One of the central features of this harshness is the concept of relevant conduct which increases the punishments allowed and lowers the burden of proof required to punish someone for conduct.
We were all taught in school that in order to convict someone of a crime, the government needed to prove its case beyond a reasonable doubt. (Obviously exceptions exist for plea bargains, where the defendant gives up the right to a trial and to have the government prove its case in exchange for a lesser punishment.)
But in the federal system, that concept - beyond a reasonable doubt - does not exist any longer. That's because the concept of "relevant conduct" in the United States Sentencing Guidelines, allows the judge to punish the defendant for uncharged crimes, or crimes for which a jury acquitted (found not guilty!) the defendant, when sentencing the defendant following the conviction on even tenuously related charges.
So, for instance, if a defendant goes to trial and is acquitted by a jury of his peers of trafficking, he may be punished for trafficking by the judge if he's convicted of any tangentially related charge by the jury.
The system is so absurd - it fundamentally turns the putative Constitutional 5th, 6th, 8th and 14th Amendments on their heads - that it creates a system where about 98 percent of all defendants plead guilty as part of agreements out of fear that even if they were to win everything (except a minor related charge), they would be sentenced by the judge in spite of those acquittals.
You'd think that such a system would be ruled unconstitutional, but the United States Supreme Court has ruled it constitution against all logic and reasonable interpretation of the constitution.