I think the principle is based on volume purchasing. I don't have access to MassDrop's purchase history with SP, or their purchase agreements, so there's no definitive way to see how the numbers "work". MassDrop isn't a clique, and viewing business transactions like they were high school social interactions feels to me like choosing entirely the wrong metaphor.
I mean, the notion of pricing according to quantity tiers is not a foreign one around here. MassDrop has the benefit of earning volume discounts, not on just each keyset project itself, but collectively on all the keyset projects for a year (or a quarter, or whatever). That probably earns them a whole other tier of discounts that no single project is likely to earn (unless your project is called Granite).