Luck is always a variable. A minor one, to be sure, but it represents the possibility of a random event that can have a radical change in events. There are things that you can't plan for- you can make plans that will adapt and help mitigate them, but sometimes Murphy happens.

Theoretical case:

You've set up camp, and will be there for another five days. Your overdue is the evening after your trip is to end. You get stung by a bee while going to your cat hole 100m from your camp, where you left your gear. This is a new experience for you, quite novel, it has never happened before. And you didn't know you are quite allergic to bee stings. Your gear means doesn't mean much right now. Neither does your skill. And your plan for calling out the cavalry... yeah, meaningless right now.

There is a chance you'll survive. There is a chance you won't. Luck, pure and simple random chance. This is an extreme theoretical, but I can't imagine that it hasn't happened. I've never been stung by a bee, and I've accidentally stepped on them, sat on one, swatted at them with my hand, even found a hive in the wall of an old out building I was helping to tear down. I could get stung tomorrow, and keel over and turn blue five minutes later. Or I might not, don't know. *shrugs*

Luck.