It seems that SDM is experimenting to see what style and frequency of offers are most effective.
I've noted in the past few weeks some people here who get 3000 wus $10 food/snacks or snacks/candy and offers of 8000 wus $30 type of sffers state they wish they had the bigger threshold offers like 16000 wus $60 and 22000 wus $100. At same time, see some getting the bigger offers expressing desire for smaller threshold offers (I am one of the people wanting smaller). Makes me think some of the strategy could be giving smaller offers to people who do big shops so that they get less points for more spending. On flip side, might be trying to entice people like me, who normally do small shops, to spend more, as the points reward so appealing.
I still think the value is amazing, and that Optimum is much more rewarding than PC Plus, and to me, the overall quality of the Optimum offers remains very high.