In a legal senses, A "life sentence" has a statutory meaning (usually 25 years as a punishment, though other jurisdictions may have different rules, typically common law legal systems have Life = 25 years). Where as a natural life may exceed 25 years to a degree of time, a criminal sentanced to a life sentance when he is 25 years of age would be out at 50 years old, where as a sentance for his "Natural Life" says that he will be in jail until he dies at some time in the future, be it 75 minutes after he is incarcerated or 75 years after.
Additionally, Parole is not part of the time in jail sentence but consideration for an early release from the sentence upon a determination by a board. Both a Life and Natural Life sentence could in theory be paroled (i.e. in the case of Natural Life convict may secure a Medical Parole or Compassionate Release as his health has deteriorated to such poor conditions that suffering illness in jail is needlessly cruel and could be released until such time that their health improves Or they pass away, in addition to a typical parole). Parole can also require a set period of time of a sentence is served. If in a statute life sentance, the parole could be a divisible number of the original sentence (if life = 25, then it could be half the time of 25 years (12 years, six months) must be served OR the sentence is set to a fixed period (both 25 years and Natural life must serve 10 years of their sentence before eligible for parole. Both a 25 year statutory sentence and a natural sentence will have a point where an inmate has served 10 years).
Anther condition could exist where a criminal is convicted of four years of a statutory life sentance, which would mean 25 life periods. He may still be elligible for parole on some but not others, such that if he gets parole on three, he must still serve the full 25 years on the fourth charge. Additionally, he can be sentanced to serve all four charges either concurrently (A period of 25 years counts for all four charges, and you are released) or consecutively (A period of 25 years counts for one charge, and when completed, the clock starts over on the next charge, and repeat until all four charges are paid). The ability to parole on each charge may factor into the sentance as suppose three charges are given a parole period of 10 years from start of service and granted parole, but the fourth charge is not given a parole possibility BUT is later overturned after the 15 mark in jail, this can change the total time served. If we assumed all three paroles are granted on first parole hearing, then this convict would serve 15 out of 25 total sentenced years as all three parole-able charges are prolled in 10 and the state acquits the final charge five years later. If consecutively, than upon the parole of the first charge, the second starts and last 10 years before it is paroled... in that time the acquittal from the fourth charge comes down, but he must still serve at least 10 years on the third charge, meaning that he serves 30-45 years out total 100 sentenced years charged and order served.