You can be someone working your way through college/grad school and surviving off of a part time job, pulling down 12k or so - graduate and start making 40k - bingo - a jump.
Or you don't work in college, start work in September and make 10k for the year, but when you actually work for a full year, you make 40k.
Then you have peoeple who were laid off, who go back to work and make good money.
Marriage often equals a quintile jump, even if you alread make a good living.
If you're making 60k, you're in Quintile 3 - you marry someone who makes 40k - and know you're in #5 - as in the top 10%.