NEW YORK (CNN/Money) - Got to hand it to the criminal justice system. Last month, a federal judge rejected a plea deal calling for Martin Grass, the former chief executive of Rite Aid Corp., to spend up to eight years in prison for his role in a $1.6 billion accounting scandal that rocked the No. 3 U.S. drugstore chain four years ago.

U.S. District Judge Sylvia Rambo thought the deal was too lenient. On Thursday, however, she sent Grass away for 8 years.

A contradiction? Yes and no. William Jeffress Jr., a partner with Baker Botts who defended Grass, said the confusion stems from U.S. sentencing guidelines.

Under a plea deal cut with prosecutors last year, Grass agreed to plead guilty to conspiracy to commit accounting fraud and conspiracy to obstruct justice. Grass also agreed to cooperate with prosecutions of other ex-Rite Aid executives in exchange for a sentence of up to eight years.

The more Grass cooperated, the greater the chance he would serve less time.

In April Rambo signaled her willingness to credit Grass for his assistance but wasn't willing to lop time off of the maximum 8 years called for under the plea deal. Grass then agreed to serve up to 10 years in prison. Rambo arrived at 8 years after crediting him 2 years for his cooperation.

"If [Rambo] had given him two years' credit [under the 8-year deal], his sentence would have been six years," explained Jeffress. "She felt the sentencing guidelines required her to start at a higher point."

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Kim Daniel, the Assistant U.S. Attorney who prosecuted Grass, did not immediately return a call seeking comment. Reuters reported that Thomas Marino, U.S. attorney for the Middle District of Pennsylvania, said Grass, 50, was also sentenced to three years supervised release following his prison term. Grass also agreed to pay $500,000 in fines and forfeit $3 million.