It's likely she only made the admission when she was offered the deal.  Unless they feel they have a very strong case, prosecutors will often take what they can.  In this case, the offer was probably that she plead to robbery in the first degree, a class a felony, in exchange for testimony against her codefendants.  Since that carries a mandatory open 20-year term, they likely said they would recommend release on parole after seven years, but the actual amount she serves before parole would still be determined by the parole board. They could make her serve closer to the full 20 years.