I’m pretty sure the prices are based on the projected sales using industry knowledge and tools like SteamSpy, created by Epic’s head of the publishing strategy at the time. It’s not common that a publisher participating in a giveaway would get to use their own figures from a prior giveaway to change the price offered by Epic, while the others’ figures are available only for the games in those leaks. In other words, claiming many copies in the present is extremely unlikely to have any effect on the future buyout prices.