Level Up (A5E) Misunderstanding something about encounter building

[neobolts]

I'm building a ECR3 fight using zombies for a 3rd level party. So if I was using all zombies, I would think it would use 12 zombies (CR 1/4 * 12 = 3). However I noticed something that made me unsure... The sample Zombie Encounters for CR 3-4 (MM p434) indicate some CR 3&4 solo mobs, but for a group of zombies lists 1d4+4 zombies. Am I misunderstanding how large groups work?

 

[neobolts]

I think I found my slip up. It's the "Low Level Adventurers and Low CR Monsters" rule. I didn't double the CR for the fractional monsters. I'll show myself out. lol