It doesn't take into affect that you can at higher levels control more zombies.

c 'control undead' juju
You are already controlling as many creatures as you can!

gr
Haelior's group:
< NPC > a zombie of the tainted dwarf lord 64% hp 100% mana 100% mv 0 xp
< NPC > a zombie of a gorgeous silver-haired elf 100% hp 100% mana 100% mv 0 xp
< NPC > a flesh golem 100% hp 100% mana 100% mv 0 xp
< NPC > another greater undead 100% hp 100% mana 100% mv 0 xp
<40 Nec> GreaterNecro 100% hp 89% mana 68% mv 298253 xp