What is the difference between Oculus OpenXR Backend and Unity OpenXR Plugin?

Hi, I am confused of the above question. The Meta document says they are not the same and the Interaction SDK supports OpenXR backend. When I use Oculus XR Plugin in Unity XR Plug-in managment together with Oculus integration to run Windows program, everything is OK. However, when I use Unity OpenXR Plugin with Oculus integration, the program also works.

Since Interaction SDK only support Oculus OpenXR backend, why can it work in Unity OpenXR Plugin?