Lower Reaches - Fireball fails to destroy 6 gibberlings

why would a fireball from a wand of fire fail to destroy utterly a group of 6 gibberlings in the Lower Reaches below the Cult of the Unseeing Eye as part of the retrieval of the first half of the rod.?

lettuce go back to Baldur's Gate 2: Shadows of Amn (original; sold 100,000+ copies) and either:
A. leave it there
or
B. call the original coding team to make any upgrades or improvements "if necessary" because

"if there's a problem with a fireball and 6 gibberlings, there are 1000 other problems"

a Fireball does 6d6 pts of damage

the lowly gibberling only has 8 hp

"2 out of 6 gibberlings were still alive with 1hp or something"

the game is unplayable.

update

apparently, 4 of them were mutated gibberlings at 16hp
low and behold 2 of them rolled crit 20 and made save vs. wand for 1/2 dmg
with their lowly saving throw value of 16 save vs. wands
I think you need to roll 17 or higher to make the saving throw rolling a d20
"it's a gibberling. it's not going to make the saving throw."

record

Mutated Gibberling: takes 23 fire damage
Mutated Gibberling: Death.
Gibberling: takes 19 fire damage fr
Gibberling: Death.
Gibberling: takes 29 fire damage fr
Gibberling: Death.
Mutated Gibberling: takes 26 fire damage fr
Mutated Gibberling: Death.
Mutated Gibberling: Save vs. Wand: 20
Mutated Gibberling: takes 15 fire damage fr
Mutated Gibberling: Save vs. Wand: 20
Mutated Gibberling: takes 13 fire damage fr
The Party has gained experience: 15
The Party has gained experience: 35
The Party has gained experience: 35
The Party has gained experience: 15

remaining Mutated Gibberling: takes 8 missile damage fr
Mutated Gibberling: Death.
The Party has gained experience: 15

remaining Mutated Gibberling: takes 3 missile damage fr
Mutated Gibberling: Death.
The Party has gained experience: 15

PS
Do You Know What a DUNGEON MASTER Looks Like.?