`cancelPerformSelectorsWithTarget:` vs `cancelPreviousPerformRequestsWithTarget:`

Question

I am building a kind of simulation with ever-smaller intervals between "ticks":

- (void) simulationTick {
    if (self.currentTick >= kNumberOfSimulationTicks)
        return; // recursion anchor

    // ... do stuff ...

    self.currentTick = self.currentTick + 1;
    [self performSelector:@selector(simulationTick) withObject:nil
               afterDelay:2.5 * pow(0.95,(double)self.currentTick)]; // acceleration
}

When leaving the Simulation screen, I want to stop the simulation, so I do this with

[NSObject cancelPreviousPerformRequestsWithTarget:self]

I first tried using

[[NSRunLoop currentRunLoop] cancelPerformSelectorsWithTarget:self]

… but this didn’t work.

Why not?

Apparently there are many others who had the same problem and the same solution. But nowhere could I find exactly why. What’s the difference between the two?