Taking my example out of context doesn't help your case.
Farmer Joe grows house-sized pumpkins. You demand to know why. Maybe he uses magic fertilizer, maybe it is because he learned druid magic, maybe because his family has fought and bled for the land for seven hundred and seventy-seven years he has a unique connection with the land, maybe he made a deal with a magic rabbit, maybe he was blessed by a pixie tribe, maybe he is the great-great-great-great grandson of an agricultural diety, maybe his family has selectively grown and cultivated large pumpkins for two hundred years, maybe he just is a really good farmer and the plants like him.
The problem is, DnD deviates in all of the possible ways. Fey contracts, special bloodlines, unique training methods, magical lands, cosmic events, blessings from gods, curses from demons, pyschic powers. You keep sitting down and demanding the ONE explanation, because the explanation that "in this world, humans are capable of incredible feats" which is a true fantasy staple, isn't enough for you.
Maybe he grows pumpkins for Giants and they taught him a secret song which he thinks is magical, but the real truth is he has been replanting seeds from the largest pumpkins for hundreds of years and the original pumpkin seed was splashed with fey blood when they were at war with the giants. Who knows! Any answer is possible, why should we limit it to single, specific answer. This is a fantasy world, people are capable of commanding gods if they speak the correct words and can freeze time with their emotions. If you put the right words in front of the action.