$ref used incorrectly?

I am very confused about the use of $ref in the schema. It seem to have been used in a non standard way.

The need to build the schema seems unnecessary, it would seem far simpler and more standard if each part of the schema referenced other parts by filename.

More importantly from what I can see the paths for the references are wrong. For instance the about schema references extensions however the path for extensions in the build schema is not at #extensions.

I hope somebody can explain, at the moment I'm very puzzled how this schema can be used.