head to head
Splatoon 2 asks more than Grand Theft Auto V, and the gap is reads.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Execution · Practiced Mastery
A team shooter where aim and positioning against human opponents controls the map.
Grand Theft Auto V leans Execution (Practiced Mastery); Splatoon 2 leans Execution (Practiced Mastery).
Splatoon 2 leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Splatoon 2
aim and movement decide ink battles
Splatoon 2 leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Splatoon 2
predicting human opponent positioning
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto V
Some mission and lobby randomness.
Splatoon 2
weapon and map rotation randomness
Splatoon 2 leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Splatoon 2
loadout and turf strategy solvable
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.