The system is based on encounter difficulty, so the relative difficulty of killing goblins for a high level character is negligable than that to a low level character.
I think it is fairly accurate... one has to remember that only because the difficulty of the enemy is static, it doesn't mean your character can't have more or less difficulty with them. For exampe, my +4 to will saves made a bodak (easy) encounter the most difficult one I've had yet because of the insta-death ability (which I thought they removed for 3rd edition?). Meanwhile, I've hacked through a few roomfools of hill-giants without even being hit (32 AC baby!
).
So difficulty is dependant on your character type as well.