Recently discovered the game Pinapple on Pizza, a free game which has an achievement for "Play" that had a 100% unlock. At the time of testing, there were 2,668 reviews and the game had been out for approximately 9 hours. I was unable to find a way to review a game I do not own, so it's safe to assume all those reviews have the game tied to their account, even if they may not have actually installed or started it.
Before adding to account, before installing, and after installing, the percentage for "Play" remained at 100%. I started the game just to see how the achievement worked. It does not unlock at the main menu, but rather after immediately after clicking "Play". Without data on number of owners, installs, and starts, it's not possible to give an exact answer. However, given how new the game is, it seems likely that a decent percentage of players would be installing or waiting to play it, and given the fact that user interaction is required to get the achievement, it is probably safe to assume percentages are based on number of players that have actually started up the game.
Previous example, which has more data to work with
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.