Crossover cutoff frequency are there to filter frequency not to pass to your speaker, however it is not cutting off frequency totally after the set cutoff, it has a gradual reduction in db.. e.g if your LPF cutoff frequency for the sub is 60hz meaning only 60hz is going to yor sub but it still plays some tones a bit higher than 60Hz but is controlled by your slopes. If you set at a -6db slope it will reduce at a rate of -6b/ octave, -12db/oct and so on and so fort..

This is a feature to control cancellation on crossover frecuencies of two drivers like tweeter and midrange/ midbass to sub.. An RTA is used to see the cancellation of frequencies, most often it can be corrected by adjusting the slopes on cutoff frequencies.

Some crossover and amp put them in degrees of phase shift to control your slopes.

HTH