A crossover doesn't just block frequencies past a certain point, it gradually (or rapidly) slopes or rolls off the dB levels past the cutoff point. A 48 dB "roll off" would be an extremely abrupt cut off, a 3 dB slope would be a very gradual roll off.
Just look at a frequency response graph for a full system and you will see how the slopes allow for your midbass drivers to pick up where your subs leave off, (usually at around 80-100 Hz or so) and then where your midbass drivers roll off and the midrange drivers pick up, and then where your tweeters pick up where the midrange left off...