$a and $b are getting values like 1 and 2 and then 3 and 4, pairs from the list.

The goal was to shuffle the numbers into a random order, so you don't necessarily want to use the values in the 1..20 range for a comparison.

Instead, the code is generating a random number and comparing it with .5, for a coin-toss-like chance of one thing being sorted above or below the other thing.