It is more than likely state law. Although our percentages are different here in CA the principle is the same. Violent felons and sex offenders serve 85% of their time. A strike offense means you serve 80% of your time. Any other crimes will get you 50% time. This applies to state prison not jail where you are credited for 6 days every time you serve 4. I'd be willing to bet that all this is set forth in the GA criminal code.
In Georgia, prison overcrowding has lead the Department of Corrections in partnership with the Board of Pardons and Parole to create a grid-like chart to determine what percentage of your State prison sentence will be served before eligibility for parole.
The chart is divided into 8 crime severity levels -- the more serious crimes require more time in jail, obviously. Also within each severity level, there are three tiers (Low, Medium, High) of time required depending on the person's probablility of success of parole as determined by an analytic chart which looks at prior offenses, prior incarcerations, drug abuse history, and the like.
By way of example only, a first time Sale of Drugs offense is a severity level 2, with the three tiers set at 18, 20, or 22 months to serve before parole. The best scores for parole success will get you only 18 months in jail, even if the judge sentenced you to 5 years in prison.
Some offenses, like Aggravated Assault or Burglary of an occupied residence, are so severe that the Parole Board moves to percentages: 65% to 90% of the sentence imposed by the court.
Still other crimes, like Armed Robbery or Aggravated Child Molestation, are so severe that no parole is possible unless the sentence is Life in prison.