[curt.clifton]

It depends.

If you have a start date but no due date, then the next start date will be as you suggest.

However, if you have a due date, then the repeat step is used to calculate the due date. The start date is kept relative to the due date.

For example, suppose you have a task "Mow the lawn". The task has a start date and a due date. Let's suppose the due date is 3 days after the start date. If you want to start the task 7 days after you complete it, then you need to set the repeat interval to 10 days.