You're gonna have to be more specific here dude.
If it's writing off stuff that has a noticeable CR difference, some of it is due to difficulty. A level 6 or 7 party can mulch pretty casually a warband of goblins even if outnumbered 6:1 and with worg support. A big batch of CR 1/3s is still 1/3s, even if the GM tries to be a rat-bastard with them. Some of it is due to action economy too, because big fights grind; they can genuinely take hours, especially if the dice are being cunts with to hit or with damage output. This is with people who know the rules.
If it's writing off lower level monsters but not too low, that is a little stupider, since it really does depend on what kind of party and what type of monster you fight. Chaos Beasts for example are far nastier than their CR 7 nature implies, as are Giant Crabs. Conversely, there are some beefy monsters that are more shit than you think. Factor in party style and strength and it gets messy.