It has to do with the way that sentences are enhanced after a conviction.  Probation officers are assigned to research the &quot;facts&quot; of a case after a conviction and write a Presentencing Report for the judge.<p>Federal sentence ranges are computed using a grid system.  Different &quot;facts&quot; adjust the row or column of the eventual range of months a judge legally has to sentence a person within.  Probation officers can be extremely thorough and throw in all kinds of things, or can omit things in the sentence computation.<p>If a person makes a plea agreement with the Prosecutor, they are essentially saying that such and such facts are relevant and these other ones don&#x27;t apply to the sentence.  If there are any possible enhancements not nailed down in the agreement, the Probation Officer can come in and say these are relevant and surprise! Your 5 year plea just became a 15 year sentence.<p>(edit) Also, multiple convictions for crimes typically have the sentences run consecutively.