No. The indiviso is the standard, default method, but a developer can set up a regime with a different allocation formula instead. What makes an approach legal isn’t the formula itself — it’s whether it’s actually written into the registered master deed (Regimen) and Rules and Regulations, either from the start or through a proper amendment. If a community is billing dues or assigning ownership share under a formula that doesn’t appear in those registered documents, that practice is illegal, however long it’s been going on.