Difference between "net 5" and "net 30" in the field "due date"

Difference between "net 5" and "net 30" in the field "due date"

Hi! I have some customers with the Payment terms "net 30". Recently have noted that for the field "due date" for this customers fills automaticaly with next logic: date of creation + 29 days.
For another side, I also have some customers with the Payment terms "net 5". But  for  field "due date" for this customers fills automaticaly with another logic: date of creation + 5 days. So, for them I must change "due date" manually.
Is it a program error?