head to head
Halo 3 asks more than Halo: Combat Evolved, and the gap is reads.

Execution · Pure Mechanics
Trilogy capstone whose equal-start arena multiplayer defined a console generation.

Execution · Practiced Mastery
The console FPS landmark that introduced Master Chief and the ring world.
Halo 3 leans Execution (Pure Mechanics); Halo: Combat Evolved leans Execution (Practiced Mastery).
Halo 3 leans a little harder on execution, though the gap is small.
Halo 3
Precision aim and grenade-melee timing
Halo: Combat Evolved
Aim, grenade timing, and vehicle control
Halo 3 leans clearly harder on reads.
Halo 3
Equal-start fights reward predicting humans
Halo: Combat Evolved
Multiplayer duels reward reading humans
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 3
Equipment pickups and spawn variation
Halo: Combat Evolved
Covenant AI varies tactics slightly
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Halo 3
Power-weapon timers and map control
Halo: Combat Evolved
Weapon sandbox knowledge and encounter routing
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.