This is purely a guess, but I'd imagine they boost posts (aka pay to display a post to more people), and show them to people who already like the page to collec

This is purely a guess, but I'd imagine they boost posts (aka pay to display a post to more people), and show them to people who already like the page to collect initial CTR/Like/Share stats. Then take the winner of that test and post to the rest of the people who like the page, where "organic reach" takes over.