How? Under the federal sentencing guidelines which allow the judge to consider conduct for which one is acquitted:

The jury's verdict notwithstanding, the judge decided that Magluta was responsible for the homicides and sentenced him accordingly. In a watershed 1997 opinion, the U.S. Supreme Court ruled that federal judges, in imposing sentence, may ignore jury verdicts of acquittal and determine whether defendants have done wrong. The Herald applauded the punishment, and the new U.S. attorney claimed that such a sentence sends a message about justice. It does indeed: The message is that prosecutors can lose and still win, that a jury no longer stands between an accused American and a life sentence.

From the Herald article:

Magluta's lead attorney, Jack Denaro, raised those issues again on Wednesday, arguing that his client deserved about 13 years' imprisonment. He decried her new sentence as ''unreasonable'' and ''unconstitutional'' under federal guidelines, comparing it to the penalty for Magluta's former partner, Falcon, who pleaded guilty and is serving a 20-year prison term. ''Even Einstein would not be able to calculate the net disparity of two centuries,'' said Denaro, who plans to appeal. ``How can Magluta, with this record, be 10 times worse?''