To explain this, we have to go over what 'parole' means.

For a life sentence, an inmate must serve at least 25 years before being eligible for parole. For lesser sentences, they're usually eligible after serving a certain minimum number of years.

However, life without parole is just that -- the inmate is never going to be granted parole. That does not, however, forgo the possibility that their sentence may be commuted (changed to a lesser sentence).

If the sentence is commuted, the inmate must remain on good behavior for the remaining portion of their sentence, or the commutation may be rescinded and the original sentence reinstated.

Even if they're eight years into the 'plus ten years' portion and are almost guaranteed to be released on parole in the next six months, they risk going right back to life without parole if they misbehave during that time.

It's basically a way to ensure that they're not abusing the privilege of a commuted sentence just to shorten their time behind bars.