To the main topic, why 90 instead of 512. It's possible at first they considered having apartments as their own unique district apart from the main divison/sub-division of the current housing system(In the same manner personal housing and FC housing were suppose to be separate)

So there option might of been introduce 2 instances of 512 or place apartments into the current instances and make it 90.

So it would be 270X20=5400 compared to 3072 if they made apartments separate, and only two instances.

The other positive about the way they are introducing apartments as a 90/90 within each instance is that when they do add more housing it also adds more apartments.

In terms of infrastructure it makes more sense to have a system in place in which adding new housing instances adds for all housing concepts instead of making apartments separate from current housing and making it difficult to balance when and how to increase the quantity of the different housing options.