The answer is randomness. It's largely random which submission of a story gets liftoff from /newest. That's why we allow reposts—we want good stories to get multiple cracks at the bat.

I realize it's frustrating when your submission 'loses' and someone else's 'wins', but since it's random, it at least evens out in the long run if you submit lots of good stories.

Eventually we'll implement some sort of karma sharing to take care of this.