TimTumThat's not true, only the players that got at least one achievement are included: "It seems this API allowed users to view the percentage of players who complete Steam achievements by up to 16 decimal points." So that means actually 27 million players played Unturned.
I think you misunderstood. A common multiple that produced a whole number for all achievements was found in order to determine the total player count.
For example, Half-Life 2's achievements, people who didn't get the crowbar would still be included in the count.
This is because of how percentages work. If 46.7% got the achievement, that means 53.3% did not.
If 8877286 * .467 produces a whole number for the number of players who got the achievement (within a certain threshold), and a whole number is produced for every other achievement percentage with that estimated player count, that means that's the total player count.
Of course, it's not clear what data Valve uses to determine the total player count for the purpose of achievement percentages, but it's around the amount of players who have launched the game once after achievements were added.