@BlackElite
it seems that he wasn't reincarnated in the same world. He makes a clear distinction among the three worlds and there are creatures and skills that he isn't familiar with, and though that by in itself isn't proof that it's a different world, he hasn't made any correlation with any of the monsters he's fought before and wonders how he can reacquire skills he once had i.e. they don't seem to operate in the same way they do in his previous worlds. How this is true, we won't know since he doesn't really go into detail about how he previously used magic and swordsmanship other than constant practice.
He starts out without any active skills aside from Analyze and 2 passives. The only reason he's been able to gain skills immediately after being exposed to them is because of his 2 passives. That said, there appears to be conditions required to acquiring skills different from how he acquired it in his previous lives which he notes a few times in the beginning.
You could compare it to playing a new game with similar mechanics to another unrelated game. It plays similarly, but not altogether the same. Like say playing DotA then moving onto HotS. Even if you've already got the mechanics down, there are some skills and knowledge that doesn't translate immediately into the other game.
Well, at least, that's how my head-canon goes so take what i said with a grain of salt. It's not focused on world-building so I don't really expect a great amount of detail put into lore. It's more like a pokemon thing where no one really cares that the protag is a 10 y/o wandering around the country fighting violent organized crime in desolate places, active volcanoes, secluded areas, illegal gambling rings, and whatnot.
I just come here to watch them do whatever and chill.