I'm sorry, what?

For me the real question is why is there an achievement for starting the game?

I'm the first with several unopened games on Steam, but come on, let's not dilute the meaning of achievement

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.
- Steam provides analytics already
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
- people who liked to keep an even achievement score
  Wait, what? Was this some kind of meta game people played, or just some weird compulsion like having to have the volume set to multiples of five?
Well, to some people the biggest achievement is getting out of the bed
- If you need to get out of bed to game, and getting out of bed is an achievement, you are doing your life wrong. Play from bed.
Achievements have honestly been diluted for over a decade
- 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

https://steamcommunity.com/stats/1363080/achievements huh? the numbers show differently (for me, at least) on the page.

It was like that in the OP, recently got fixed. :)

I booted the game up and so far the only achievement I have is the first year achievement.

No idea if it's a glitch that has kept me from getting "start the game" or what but I haven't gotten it yet

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.

On steam it's always a percentage of the people who bought the game/added it to their library
- It actually only includes people who launched the game. This achievement essentially shows how many people actually clicked play after launching.

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.

Haven't heard of pirated games with steam achievement support, like how would this even work, you either have the game in your steam account or not
- 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

Could be the second one a more recent achievement?

More people are puzzled by this achievement https://steamcommunity.com/app/1363080/discussions/0/4355620138226996074/

I don’t know as “fart into a glass of water“ is a real achievement, or at least not one I’d admit to.

There must be a lot of speed runners

No one's rang the church bell yet?