Ask Game Masters

Yeah, that's a great tool to use as a starting point. But getting a good looking number from that calculator doesn't mean that your party isn't going to smash the enemies or get wiped in three rounds. Different monsters will have different abilities (AC, attack bonus, damage, spells, resistances, saves, etc) just as your PCs will depending on their classes and builds. A group of slow moving monsters with high AC and low dexterity saves might be an appropriate or tough challenge for a heavily martial party, but add one wizard with spells like fireball or even burning hands at lower levels and suddenly they're all taking full damage at once. There's also the action economy to consider; a single enemy that attacks once per turn isn't going to deal as much damage as a party of four all attacking it (unless it has legendary actions or it's hitting hard enough to drop a PC in one or two hits) but a bunch of low CR enemies can potentially take out a half the party in a round or two via sheer volume of weak hits if they aren't hit with some sort of AoE attack. As the DM you have to adjust for such factors and that's really much more of an art than a science.