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.