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
Bit of each for different people I suppose, but for most probably closer to the latter, most achievements being worth a multiple of five making ones that don't stick out. Checking mine, currently ends in 4 so must have run into a game that did something like that at some point, but am not one of the people it bothers