Kills score error on GBA Doom II map 05, The Waste Tunnels:

A "KILLS" score below 100% can occur on several maps in GBA
Doom II, even when the player kills every living monster.
This is largely due to a flaw in this game's score system.
In the original PC Doom games Lost Souls are not considered
as score-affecting enemies, and the player will not receive
any credit for killing them.
Lost Souls in GBA Doom II are considered as score-affecting
enemies but, as in the PC games, the player still does not
receive credit.
Another difference in the GBA Doom II Kills-scoring system:
monsters killed by other monsters do not count towards the
player's score.