Because someone in the gamedev community once posted about using achievements to measure engagement with users, it caught on, and now there are actual achievements mixed in with standard progress, including just launching the game, apparently.
They are used to measure how far players get into the game so you'll often see one for starting the game the first time, completing the tutorial, and chapters as well as difficulties. The reason the numbers are off here is likely that achievement was added some time after the game released and those players never returned to the game and launched it again 5o earn that achievement.
I remember some games on Xbox did that in a way that fucked with people who liked to keep an even achievement score, award a nonstandard two point one for starting the game and lock the "corresponding" three point one behind something endgame with the rest being standard multiples of five or whatever
Strictly talking about statistics, it depends on the context. From everyone who bought the game, 39% started it. From everyone one who played the game (so started it) 79% completed the first level.
This could be just a way to track pirated copies. First achievement comes from Steam statistics where the second one is from the game itself. This way you can track how many copies of the game are being pirated.
Not Steam, but interestingly some Microsoft published games will unlock Xbox achievements when pirated, presumably because they built the version sold on Steam to unlock achievements on both platforms. Pirated Psychonauts 2 and got the little under half you get on a casual non achievement hunting playthrough on my Xbox account