The sentence you receive from a judge is probably not anywhere close to what you will actually sit in jail for. Probation, parole, double time, triple time, good behavior, trustee status, all affect the outcome.
For example, you are sentenced to 3 years of jail for a non-violent drug offense. How long do you serve in jail? Very commonly 366 days, though it may be as much as a year and a half. That is if you don't get into any other trouble in jail. After that point you will be released on probation.
What the previous poster is likely stating is that if you willingly agree to be a slave for the state you get all the benefits of early release. The defectors that do not want to be slave labor are not given said benefits. An ironic application of the prisoners delma. If none of the prisoners would be willing to be a slave, the state could not hold them all, or 'extend' their sentences because of overcrowding.