I shall take the assumption that based upon your good Scottish username that you are in fact Scottish. The reason for this is (as far as I understand) because the government didn't want to pay for two overlapping schemes at once, and prefers to operate independently without such schemes being imposed upon it. Also, the scheme is broader than the 16-17 Saver scheme, covering 18 year olds regardless of whether they had applied for their discount before or after their journey. The discount is also automatically bundled with the free bus travel card, which also gives a wide range of discounts itself (e.g. 10% off at Scottish Co-ops), which are not available to those not resident in Scotland. Therefore, it was considered uneconomical to raise the discount to match the Saver.
I suspect this particular decision is from the UK Department for Transport, and considering their well-renowned competence likely entirely forgot about this. I suspect they themselves also don't care - not their trains, not their problem.
You would be treated the same as if you had no ticket at all. I imagine a number of inspectors would be more understanding, and therefore issue an excess to an undiscounted fare. However, they'd be well within their right to charge a minimum fare or full undiscounted Anytime Day Single, whichever is dearer.
