head to head
Mario Kart: Double Dash!! asks more than FlatOut: Ultimate Carnage, and the gap is variance.

Execution · Pure Mechanics
Keeping speed through destruction-racing chaos that punishes clean-lap plans.

Execution · Twitch Chaos
GameCube kart racer with two riders per kart trading items mid-drift.
FlatOut: Ultimate Carnage leans Execution (Pure Mechanics); Mario Kart: Double Dash!! leans Execution (Twitch Chaos).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
FlatOut: Ultimate Carnage
Holding racing lines through wrecks and ragdoll chaos tests hands
Mario Kart: Double Dash!!
Two-character swaps and drift timing
Mario Kart: Double Dash!! leans a little harder on reads, though the gap is small.
FlatOut: Ultimate Carnage
Rival drivers ram predictably, light multiplayer jockeying
Mario Kart: Double Dash!!
Item duels against humans reward baiting
Mario Kart: Double Dash!! leans clearly harder on variance.
FlatOut: Ultimate Carnage
Debris and pileups scramble any planned lap
Mario Kart: Double Dash!!
Special item rolls swing races
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
FlatOut: Ultimate Carnage
Car upgrades and track shortcuts are mappable
Mario Kart: Double Dash!!
Character pairings and lines are solved
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.