Here is my best off-the-cuff logic behind the calculation. The mortality rate is the ratio of how many people have died out of all those who have been infected. Given an active infection case where the victim is being treated, the victim has not died. In this case the active case is shown as a part of the denominator, which is the total number of diagnosed cases. The top line, the divisor is not effected, as the victim is alive. If the patient subsequently dies, the total number of diagnosed cases has not changed, i.e. the denominator stays the same, but the death count, the divisor, has increased by 1, which then increases the mortality rate.
Here's an illustration:
Total number of diagnosed positive cases: 100
Number of positive cases who have died to-date: 5
Mortality Rate: 5 divided by 100 = 5%
Total number of diagnosed positive cases: 100 (unchanged)
One more of the positive cases dies: 5 + 1 = 6
Mortality Rate: 6 divided by 100 = 6%
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