head to head
Call of Duty: Modern Warfare 3 asks more than Half-Life Deathmatch: Source, and the gap is reads.

Execution · Pure Mechanics
A fast-paced military shooter where twitch aim against other players is the core test.

Execution · Pure Mechanics
Tests raw aim and movement speed in fast classic arena deathmatch.
Call of Duty: Modern Warfare 3 leans Execution (Pure Mechanics); Half-Life Deathmatch: Source leans Execution (Pure Mechanics).
Call of Duty: Modern Warfare 3 leans a little harder on execution, though the gap is small.
Call of Duty: Modern Warfare 3
aim and reflexes decide most engagements
Half-Life Deathmatch: Source
Aim, bunny-hopping and snap reactions decide duels
Call of Duty: Modern Warfare 3 leans clearly harder on reads.
Call of Duty: Modern Warfare 3
predicting human opponent positioning and habits
Half-Life Deathmatch: Source
Predicting human movement and spawn timing matters
Call of Duty: Modern Warfare 3 leans a little harder on variance, though the gap is small.
Call of Duty: Modern Warfare 3
spawn placement and loadout drops vary
Half-Life Deathmatch: Source
Fixed maps and item timers, little randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 3
loadout tuning and map knowledge help
Half-Life Deathmatch: Source
Weapon spawns and map control are learnable systems
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.