head to head
Left 4 Dead asks more than Far Cry Primal, and the gap is variance.

Execution · Practiced Mastery
Open-world Stone Age action game about hunting and conquering tribal outposts with tamed beasts.

Execution · Twitch Chaos
Co-op zombie shooter whose AI Director reshuffles every run to keep teams honest.
Far Cry Primal leans Execution (Practiced Mastery); Left 4 Dead leans Execution (Twitch Chaos).
Left 4 Dead leans a little harder on execution, though the gap is small.
Far Cry Primal
Bow aim, melee timing, beast control under pressure
Left 4 Dead
Headshots, melee shoves, and special-infected reaction time
Left 4 Dead leans a little harder on reads, though the gap is small.
Far Cry Primal
Scripted tribal AI never models the player
Left 4 Dead
Versus mode adds human ambush mindgames
Left 4 Dead leans clearly harder on variance.
Far Cry Primal
Wildlife encounters and patrol spawns are random
Left 4 Dead
AI Director randomizes hordes, items, and specials
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Far Cry Primal
Outpost routes, crafting economy, beast taming order
Left 4 Dead
Team spacing, route choice, and resource discipline
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.