head to head
Spelunky asks more than Mario Kart 8, and the gap is execution.

Execution · Twitch Chaos
A kart racer where controlled chaos from random items tempers otherwise precise driving skill.

Execution · Twitch Chaos
The original procedural platformer that codified the modern roguelike action loop.
Mario Kart 8 leans Execution (Twitch Chaos); Spelunky leans Execution (Twitch Chaos).
Spelunky leans clearly harder on execution.
Mario Kart 8
precise drifting and item timing matter
Spelunky
Tight platforming and whip control under constant threat.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mario Kart 8
minimal, mostly reacting not predicting
Spelunky
Enemy AI is scripted; no adaptive opponents.
Spelunky leans a little harder on variance, though the gap is small.
Mario Kart 8
item boxes are heavily randomized
Spelunky
Procedural caves and item rolls shape every run.
Spelunky leans a little harder on optimization, though the gap is small.
Mario Kart 8
track lines and item strategy solvable
Spelunky
Hell route and shortcuts reward solved knowledge.
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.