I probably can not explain it properly in mathematical terms. I do know that four 10% does not equal a net 40% change. It just can't.
If you have a 100 gallon tank and change 10 gallons today, you have done 10%. If a week from today, you do another 10 gallons, that is 10%, but some of the water you took out was some of the new that you added last week, so maybe its more like 8% net (who knows?). Continue losing 2% for weeks 3 and 4 and you are probably closer to a net monthly change of 33-35%. That is not taking into account that smaller changes do less to bring things like nitrates down. I'm a bigger fan of doing changes as needed and I typically do 30-40% at once. If my nitrates get high after three weeks, i do a change. If they stay low for 6 weeks, then I will change when they creep up. I am talking for FOWLR, so for reef, the nutrient replacement probably needs to occur more on a schedule.