Because rsf includes building common spaces, which tenants are in fact renting. This is not complicated.
Yes, it does appear plain that this "common spaces" business is a fig leaf that covers annual rent increases. And yes, that's dishonest. But it's of no practical import, since every tenant knows their rent is going up one way or another once their lease expires.
We're really making a big deal over whether "rent increase" is called "rent increase" or not.