I don't mean that their stories are not chronological... what I mean is that in the process of writing, they might start out writing a scene in the middle, then a scene at the beginning, then at the end, etc.
However, I've never actually met anyone who claimed to do this.
*raises hand* Now you have! And I must plead with everyone here to never start writing non-chronologically. It's a terrible habit, and one I've finally begun to break.
I didn't always write like that, though. When I began writing, I scribbled my stories on paper, then moved on to an electric typewriter. Either way, the use of paper gave me no incentive to skip around. Skipping around would mean losing my place, and likely having to eventually retype a lot of pages. It was best to just soldier on with whatever scene or chapter was giving me difficulty.
And then? Sometime around 1994, my family got a computer, and soon after that I became an Internet junkie. My attention span plummeted. My writing philosophy became "if it's too hard, or if it takes too long, skip it!" This can work in moderation. Just tonight, I stared at the last page of Chapter 5 in my novel for a good 15 minutes, tried a few sentences, deleted them, stared some more... and finally realized that if I was going to write more than 50 words for the day, it was best to just move on to the beginning of Chapter 6. So I did. 500 words done. But I didn't dare move on to Chapter 7, or 8, or 18. Not anymore.
The worst thing I did, back in my unstuck-in-time writing days, was jump back and forth even within one scene. I'd write the first sentence of a scene, then the last sentence, then try to connect the two with an appropriate middle. Didn't work very well. I got to the point where I rarely even had a complete sentence in my manuscripts -- I'd start a thought, realize I didn't know where it was going, truncate it before the period, and move on to another. You can rack up a word count this way, but not a story.
When I realized I was writing forum posts, Livejournal entries, and emails this way (because it's so easy to start with "Regards" or "Talk to you later!"), I knew I had to buckle down and start writing things in order again. I've spent almost a year working on this, and it's finally beginning to feel more natural for me.
In fact, I think I'll go fill in that hole at the end of Chapter 5 before I head to bed tonight.