Who counts as a "player", for Steam achievement percentages?

For each Steam achievement, the platform gives statistics along the lines of "23.6% of players have this achievement". Who counts as a "player" for this calculation? (To put it another way, what is the denominator?)

It is relevant if you want to figure out how awesome you actually are.

For example, if "players" were defined as "anyone who has bought the game", then my achievement percentages compare me not only to people who have played the game, but also to those who haven't even downloaded it! Which means that my achievement is less impressive than one's initial intuitions might suggestion.

On the opposite end, you could define "players" as something like "anyone who has played the game for over an hour". Or perhaps "anyone who has played the game for at least as long as I have".

Maybe Valve answers this somewhere, but I couldn't find it easily.

Take a look at the game Chime, specifically the achievement Fairy Godmother. It is awarded for starting up the game. No user interaction required. This achievement has a 98% global unlock.

Using data from this article, Chime had roughly 106,325 players as of July 1, 2018. This means either at least 2126 players had that achievement bugged, or Steam does not limit it to only players that have started the game.

While I've not done an exhaustive search, I attempted to find complaints for that achievement not triggering. I was unable to find anything, and I'd think at least one of those 2k+ people would have mentioned it.