He seems to have known the common language since he was a baby and even remembers being a baby in which he had mature thoughts, but he has no common sense as a ten+ year old child. It seems like he was deliberately raised differently after he was abandoned as mentioned in the summary, but to not notice his differences with other humans is weird, especially after he comes into contact with them.

I mean people are obviously surprised and go "that's impossible" and he just thinks that he failed the test because he didn't do things the normal way? What? They obviously said that what he did was theoretically impossible and he just proved them wrong so I don't get where his misunderstanding comes from unless he's mentally deficient and doesn't understand human language. Considering he's so smart he can read books on ancient theories in the ancient language, he shouldn't be oblivious, he should be wondering if his education was a little more advanced.

It could just be that he had a high IQ from birth, but they even hinted at the "he has black hair and it's an evil omen" thingy in the very beginning. They never mentioned it when he first meets other humans though and nobody looked down on him. So I really don't know. Maybe it's supposed to be some sort of mystery for the readers but the lack of common sense in a supposed genius is just really dull to me. A real genius would notice the incongruity between his standards of magic and that of others.

The real annoyance was when he checked the admittance for the academy and was, for some ridiculous reason, afraid he'd fail because he broke the examiner's sword and defeated him? And he didn't make a magic device in the creative magic test that was normal so he thought he wouldn't get extra points? That's despite doing something "theoretically impossible". He didn't check the top scores at first because he didn't think his name would be there and he almost left in disappointment before even seeing his score. At that point I stopped reading because I couldn't stand it anymore. That whole "I'm sure I'm just a normal person despite everyone and everything around me proving I'm not" thingy annoys me.

If he really is just a normal child, it could have been cute, but he's more like a reincarnator that forgot that he was one and forgot all logic and reason as well. There was no childishness to him. The author tried way too hard to make the main character misunderstand his own level of power while throwing in reincarnation story tropes.