I know that some people like to joke that the Pokédex is already filled out for you and all that, but here's an interesting theory: each Pokédex entry is written by YOU the player after catching a certain Pokémon.
That would explain why it includes nonsensical folklore like Cubone wearing the skull of its dead mother...
I've actually heard this theory a lot over the years and buy into it 110%. Even if your not the one writing the entries then the Pokédex would have to be writing itself. Having the entries already written actually breaks lore considering that we were originally given them because they hadn't been filled out yet.
The player character has such a dark thought process to come up with some of these Pokédex entries. What I find really hilarious though is that if the player is the one who wrote all of the Pokédex entries, they're just making stuff up on the spot, and Professor Oak and everybody else is going to be all like 'Oh, there were certainly a lot of Pokémon that used to be humans. I wonder if we could recreate that scientifically', and the player's just sitting there, trying to keep a straight face.
Turns out our player character is a Chaotic Neutral Gremlin who likes to troll the Scientific community thats too lazy to double check what this childs writing in their magnum opus of technology with dark enties and wildly absurd numbers. It wpuld certainly explain the numbers of temperature, and other stuff being all crazy. The question is though, did they write all of the stuff in the Pokédex or just the entries? Like did they meticulously measure out the heights and weights of each Pokémon species to find their average or did they just go. "My Eevee is 1 foot tall so all Eevees are 1 foot tall." Or maybe. "The god of the land seems about yeay big so Imma write that down."
I've seen 3 theories that boil down to You the player are the one writing the entries. The entries already exist but you the player update them with what you find. The dex was already completed via some server and said computer tracks your progress on the dex remotely adding the entries as you go on. No one can agree on this and IMHO it's just stupid. I don't think it's a topic to be deep dove into