Its £9 upfront, £100 reward and the £27.99 (x24 months) total, then that amount is divided by 24 to show the equivalent cost, but hopefully showing it this way helps?
£27.99 per month times by 24 months = £671.76
£671.76 minus £100 reward = £571.76
£571.76 plus £9 upfront fee = £580.76
£580.76 divided by 24 = £24.1983333 (so rounded up to £24.20)
The £100 reward doesn’t actually come off the monthly bill, you still pay an upfront fee of £9 and then its £27.99 per month.