See my reply to another comment. But yeah, there’s no fundamental reason. But it’s conceivable that the difference goes beyond simple subsets of fields, but entire relationships being hidden.

Let’s say in your internal model there’s a relationship A - B - C. But for client apps it makes no sense to expose B, so instead you choose to represent the model as A - C. This is more than just a simple subset. Someone who understands graph theory better than I may be able to explain if there are elegant approaches to this.