It's actually a 10 to 25 indefinite sentence, but in Nevada typically offenders are released after the minimum as long as they stay out of trouble inside (q.v.
OJ, sentenced to 9 to 33, out in 9).
But for them to get a deal like that there must have been some kind of evidentiary or other issues with the case. I can't see what there could be, I haven't looked into the case much, but perhaps something that made it other than a typical murder committed during a robbery. One issue is that they couldn't figure out which one pulled the trigger, but that really shouldn't matter, the state is allowed to prosecute both equally without having to prove which one was the actual triggerperson.