There are two legitimate explanations, and one of them is far more common than people assume.
The first and most likely is that the sentence was life with the possibility of parole, not life without. These are fundamentally different sentences even though both are labeled "life." A life-with-parole sentence means the person is eligible to appear before the parole board after serving a minimum number of years, often 15 to 25 depending on the state and the charge. If the board determines they meet the criteria, they are released. Seventeen years is a realistic timeline for someone who served consistently clean time, participated in programming, and made a compelling case at their parole hearing. This is the most common explanation and the one worth starting with.
The second possibility is substantial assistance, which means the person provided information to law enforcement that led to arrests or convictions of others, and prosecutors filed a motion with the court to reduce the sentence as a result. This kind of cooperation agreement can be arranged at sentencing or at any point during incarceration, and it can result in dramatic sentence reductions even on very serious charges. It is rarer than parole, but it happens and the outcomes can be just as dramatic.
Without knowing the specifics of the case, parole eligibility built into the original sentence is the most straightforward explanation for what happened.