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
Diluted implies they were ever worth anything. They were only created as a dopamine drip to keep customers buying their product. Using them for publicly available analytics is great. I love being able to compare the drop off rate of one game to another. Comparing how quickly players dropped of compared to, for example, Dark Souls is fun and informative.
Personally, I love having a ton of them - I wish there were more
I don't mind standard progression ones, they at least give me a rough idea how far through the storyline I am
But the high end ones are way too few - I like having an unlockable goal to work towards if I'm not done with a game by the time I finish the in game progression. I want them to be hard, maybe even require multiple playthroughs with a severe handicap. Not tedious collecting... But like in Prey when you had to play through with only human, typhoid, or no abilities unlocked. Then you had to save everyone and murder everyone - I had fun trying to combine as many as possible into a playthrough
My problem is that people get upset when 100% is a long and hard road, and a lot of games have lowered the bar to keep them from getting upset. Might as well make prestige achievements that put the total over 100% at this point... Hell if I launch something on steam I might do free DLC just for that
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
Mods usually disable achievements. And depending on when exactly "start the game" happens (Game launch? Clicking "new game" on the main menu?), it's possible but weird to get enough playtime to count for steam stats, but not actually do the thing to get the achievement.