During a case, evidence is presented and judges or juries base their rulings on the information that is given to them. This doesn’t mean that the information given is always completely accurate. Mistakes can be made which might lead to problems in the eventual sentence that is given. Since courts make mistakes, there is an appeal system to right these wrongs.
When people feel that something has gone wrong with their initial case to make their sentence unfair they turn to federal appeals court to be given a chance at resentencing. In a case worked on by the NLPA, erroneous drug amounts were brought to light. This resulted in a sentence being set aside and eventually reduced by five years because of this change.