KingCrab said:
...(than the party) monsters can often result in PC deaths, so they purposely design the creature to be more challenging, yet not do damage too any one character too quickly?
Yup. That's a lesson learned from 3E's CR-system. Also, the reverse is problematic: PCs that are higher level than the CR can often mop the floor very easily with lower CR-opponents.
That's because we have several level-dependent variables:
AC, Attacks: Describe the "danger" of the monster, because it makes it less easy to hit, and can hit PCs easier. These are static things during a combat.
Hp, Damage value of attacks: Describe the ablative effect of the monster, i.e. how long it takes to vanquish the monster and the impact the monster has on the PC resources.
Total damage dealt by the monster: That's the problematic bit - it depends on the attacks, but also on the AC and hp (because hp allows it to last longer). The total damage dealt by the monster rises faster than the other variables - because of these independent dependences on the CR/appropriate level - it's proportional to hp (and hence level) and attacks (and hence level) and perhaps AC (and hence level), meaning a quadratic or cubic rise.
However, we have to fix up the AC and attacks fitting to the PC level to get a "good" hit-and-miss rate - otherwise the monster is too easy or too hard.
This means we have to have proper, level-appropriate hp and damage dealt, but if we only increase hp, we have to decrease the damage dealt by the monster to keep the overall impact on PC resources the same.
But the CR system cannot do that, and the reverse is also true (which explains why lower-CR monsters are so easily killed off, because the PC rise in their "CR" for the monsters).
Because parts scale linearly (AC, attacks), but others scale quadratic or worse, it's hard to tie it's overall impact on the party in a single number.
Differentiating into a "level" (AC, attacks) and "elite/whatever rating" (hp, damage dealt), allows judging these different proportionalities better and hence give a better estimation of overall impact on the party.
Cheers, LT.