The way I see it, it's a tactic designed to get you to use the website and cruise planner more often. The more often you visit, the greater the chance that you'll talk yourself into getting something that you might want, but don't necessarily need.
As for the actual discounts, think of it as similar to retail stores that have a sale every week or two. This week it's Memorial Day. Next, it's "goodbye May, hello June". etc.