1x developer might look at the problem and think "this is an interesting and challenging problem to solve, I'll solve it in a week". He solves it in 2 weeks, and despite failing his own deadline feels really good about himself. After all he solved a problem.
10x developer might look at the same problem and think "this is an interesting and challenging problem to solve. I'm not going to solve it today because the impact is too low and cost of the solution is higher than cost of the problem".
1x developer might ship 500 lines of code per day.
10x developer deletes 50 lines of code per day, and ships a 100.
1x developer is good at solving problems.
10x developer is good at not creating problems in the first place.