The substrate can store toxic chemicals like a sponge - a single 90% water change will do far more than three 50% water changes (first: 10 ppm * 0.5 = 5 ppm; next one: 5 ppm * 0.5 = 2.5 ppm; third one: 2.5 ppm * 0.5 = 1.25 ppm. Now a single 90% is: (10 ppm - 10* 0.9) = 1 ppm. So a 90% W/C gives 1 ppm but THREE 50% W/C give 1.25 ppm!!!!

Note the math. It is tricky for values that aren't 0.5. For example: a 75% W/C is NOT 10 ppm * 0.75 = 7.5 ppm!!! Not correct. It is really 10 ppm - (10 *0.75) = 2.5 ppm left. The 0.5 is a special case that works by simple multi. In a similar fashion, a 25% W/C is 10 ppm - (10 *0.25) = 7.5 ppm left in the tank. Try this method for 0.5 and it still works.