Comment by Em_likes_Cho on Isekai de Cheat Skill wo te ni Shita ore wa, Genjitsu Sekai wo mo Musou Suru ~Level Up wa Jinsei wo Kaeta~ - Chapter 5

That is pretty much how everything is done.

Most anime, change stuff for a lot of reason. But the main reason? Its just how it is. If a series starts as a webnovel and blows up and get adapted to light novel? There are changes and the WN version is removed, same with LN getting adaptation into manga it will have changes, same with manga getting adapted to anime.

There are no other reason other than its just how it is for 9 out 10 series. Some are too dark like tanya the evil WN version has a far more grueseom death for Marry Suiox, the LN is far more merciful to her. In the WN she was killed off by her own squad and then dumped her body into some trench/ditch, the LN did kill her too, her squad kills her but it was in a barrack (place not confident so its IIRC scenario) and her corpse was not mistreated at all.

Same with overlord, the WN version is darker when it comes to the female adventurer that pays for her twin sister. She died, and her twin sister were sold to slavery and died getting gang Rap#d. In the LN, they were just sold to slavery and survive, the female adventurer lives but becomes the vampire NPC s** toy.

Hence it was changed. But those are specific titles with specific genre. Like I said 9 out of 10 series are just changed that way.

Always treat manga version vs anime version as a complete different version altogether, its easier that way. Same goes with the LN vs manga, or webnovel vs light novel version.