I believe the issue relates to "Breakage" .. i.e. In this case the expected non redemption of a significant percentage the reward facility or at a lower value per point.
This would have been factored in by analysts at the margins in relation to profitability.
This occurs both with the airlines reward scheme which gets pressure when more points are redeemed than factored by their analysts and the financial institutions who are subsequently pressured by the airlines who want to increase points costs.
Those who earn points in dribs and drabs are more likely to not redeem points (and if they do, at less value cost per point) than those earning in bulk, especially when in large chunks.
It's similar thing as to what happened with the Westfield XS card when they were being purchased on points earning cards and redeemed 100% - that is not what is supposed to happen when breakage of up to 30% is expected.
With Millions of Rewards points a .01% variance from predictions can make a significant dent on the bottom line.