Why are there more people in the “red” than the number of people who have been in contact with the response team?

There may be multiple explanations. The two most common ones are:
1) The employee has an open case and continues to respond every other week.  Their wellbeing does not change immediately and as a result their responses still fall in the “red zone”. Since they already have an open case, which is waiting for a follow-up interview (which is typically 3-4 weeks ahead), no new case is set up.
2) Over the past 12-month period the employee has had 2 cases (which in some cases might be a maximum number of cases per year), and as a result no new cases are set up but are instead referred to the closest manager, HR or other healthcare service.