24 db is twice as steep as a 12db...
not trying to sound condescending, but it has to do with how fast the freqs start to roll off.
per octave range (each octave is double the frequency above or below the starting octave) the sound is rolled off in either a 24 or 12 Db slope. so if you set your low pass at 200 Hz, by the time a 100 Hz freq is sent through the processor/crossover, it will be muted by either 24 or 12 Db.