did you read what you quoted? I said they are both wearing armor, yet one can withstand 2 arrows and the other can withstand 20 arrows. The armor makes no difference in that.But the 10th level fighter is wearing armor, and what would happen if they weren't wearing armor is irrelevant. If a ruleset is going to be efficient, then we need to track which assumptions are in play, and that's one of them.
How is the 10th level fighter able to take 10x as manny arrows while wearing the same armor?