Part 2 section 4.3.3 allows omitting key values that are implicitly specified by referential constraints to key values provided in previous path segments:

https://host/service/Orders(1)/Items(OrderID=1,ItemNo=2)
https://host/service/Orders(1)/Items(2)

The shorter form is preferred for parentheses-key syntax, and both are allowed.

We cannot give that choice for key-as-segment style because the number and order of path segments must be deterministic, so it is either

https://host/service/Orders/1/Items/1/2

or

https://host/service/Orders/1/Items/2

Shorter is better, so it must be the second.