If more than 40 % of braking force is required, you'll receive an audible and visual warning to intervene by applying more braking force. (mbrvc.com)
From what I know, dual - pivot ones offer more braking power, which leads us to the answer: rear wheel has a «weaker» brake. (bicycles.stackexchange.com)
And more braking in the rear keeps you from spinning, unless you just completely blow it and lock up all four tires. (mechanics.stackexchange.com)