[Luvian]

The reasoning behind the challenge rating is that you get better when you are challenged. A level 1 fighter fighting a goblin or orc might be evenly matched in skills so he might learn new tricks and get bettter at using his weapon, but a level 15 fighter is a master of fighting, the goblin might as well be a dummy.

If the game's challenge rating for PCs is the same af for npcs, then your challenge rating is higher depending on your class, level, and also your health. A level 20 fighter that got 1s on all his hp roll will have a challenge rating a lot lower than one that got all 10s.

Do you know what the challenge rating is? It's the level a group of 4 adventurers should be to have 50% chance of winning that encounter. So if a Dragon has a challenge rating of 12, to have 50% chance of winning, you need a party of 4 level 12 characters.