Mortality may appear to be higher than it actually was during a plague year if people moved away.

This is because the death rate is a calculation based on the number of deaths divided by the population. So, if people moved away, the population would be lower and so the number of deaths in relation to this lower population would appear higher.

For example:

A. Normal year

If deaths in a normal year = 100

If the normal population = 5000

Then the death rate for a normal year would be: 100/5000 = 0.020

x 1000 = 20

B. Plague year (no migration)

If deaths in a plague year = 1000

If nobody moved away so the population remained at 5000

Then the death rate would be: 1000/5000 = 0.2

x 1000 = 200

C. Plague year (with migration)

But if 1000 people left:

The death rate in a plague year is still 1000

But the population is now 4000

So the death rate would be: 1000/4000 = 0.25

x 1000 = 250

The number of people who die is no different between B and C, but because of the number of people who left the area the death rate appears to be higher.