The obligatory nisab is not the exemption - which is what is between the nisabs - for if one owns thirty-five camels, the obligation, which is a female camel in her first year, is only on twenty-five, not on the total. Even if ten camels perish after the year, the obligation remains the same. It is called an exemption because the obligation of zakat is due before it exists. See: Fath Bab al-'Inayah 1: 505.