It took me awhile but I finally figured out Marvel's reason for the new discount plateaus and admittedly, it's quite brilliant.  Under the old system, Marvel had no control over store discounts.  Marvel would offer products each month and retailers would order what they thought they needed and get the discounts they wanted.  Single and smaller stores probably fluctuated with their discounts since some months there would be less product offered than other months.  Or, Marvel may relaunch a 'hot' title and a small retailer would order much more than normal and temporarily boost its discount.

Under the new system of basing the monthly discount from the average of the twelve previous months, Marvel can now plan when to release certain titles and how many.  Since Marvel is tracking every retailer's discount, they know exactly when stores are at their highest and lowest discounts.  They can even do projections based on past ordering practices.  With this knowledge, Marvel can then predetermine when to release 'hot' books.

When retailer's discounts are low, they can offer the 'hot' book so retailers can't get quite as good a discount on all the extra copies they will stock up on.  Because of this surge, the retailer's average will rise in a few months.  Then Marvel can scale back titles or avoid launching hyped books and therefore lose less money, as the high discount will be wasted on less product.  The lower orders eventually force the discount to drop and Marvel can look at pumping out more products again at lower discounts.