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.

It lets you have analytics in your game (how many players do X, use y feature), without the backlash of analytics.

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

Well, to some people the biggest achievement is getting out of the bed