Random luck. I've tried a number of different ways and I've only drawn a total of twice in 20+ years of applying for each of the four rivers, and the draw was for the same river almost 10 years apart. The most recent draw was through the new lottery system (one lottery application per river), it was a late season date, used my fourth date preference, which had less applicants than mid season dates.
My understanding of how it works is if your name is drawn in the lottery, the system looks at the dates you chose starting with the first preference and if that date is full, it moves to second, third, and then fourth as needed. If all four dates are full, even though your name was drawn, you're out of luck.