I think that generally, the idea is that humans in Runeterra have a "greater potential" than ordinary humans. It might be down to innate magic in them or just because Riot wants things to be super amazing as sod, but humans can be as weak as normal humans or godlike by our own standards, with the idea that humans can even transcend their own physical limitations and become something more being paramount.
It's kinda like in One Piece, where humans can gain such amazing potential through training alone, without any other stimuli. There's a whole power called Haki which is essentially transcending human limitations to allow your will to have an impact on the physical world, either through six sense or being able to bring harm to intangible things. Zoro's probably the best example you see; he's super strong through just his own strength and merit, but far stronger than any human in the real world. He threw a house once. He can make an illusion with his own force of will that can actually harm people and physically damage things. He can lose more blood than humans naturally have and still survive, and he's just a human.
It's all about that human potential stuff.