I think I've got it.
My unlimited data days ended 17/11. This is the first billed month (pro rata). Spend limit is on R250.
Rain are scoring from me since I thought the unlimited off peak (19 hours) would be charged pro rata after 17/11 & I would not be billed a full month of unlimited off peak (R250). And that miraculously, that there would be no peak hours data connection.
What's happened now, is that I will be continued to be billed for peak use until spend limit (R250) is reached. And even though I'm not using a full month of unlimited off peak, I will continue to be billed for peak use until spend limit has been reached.
Lesson:
During your first month of use after free unlimited data days have ended, the spend limit needs to be set lower than the R250 required for a full month of unlimited off peak. This is also why the billing system allows for the dashboard to have a spend limit of less than R250 during first month, when there is not a full month of unlimited off peak, due to free unlimited data days at the start of service.
Then, during the next calendar month being the first full month of unlimited off peak, it is not possible to set the spend limit below R250 when unlimited data days has been enabled.
This may also be complete & utter rubbish I have just managed to come up with!