Its because openXR doesn’t have the concept of “curls”, instead of blending between a relaxed and closed pose its directly driving the transforms. I would like to estimate some curls eventually but don’t want to commit to iteration time on that until the oculus hand tracking is also functional so it can be universal.
The hand opens more than normal because the mesh for the manniquin hands are not bound exactly the same as the steamvr hands relative to each other, joint lengths, angle to the mesh, ect.
There is another avenue of using the relaxed pose as a base for additive, however then the end fully gripped pose is likely to have some clipping.
When you think about what they are estimating with the controllers, your hand fully off of the controller “should” be considered fully open like that, they have no other data to go off of, relaxed pose would normally be the hand slightly closed around the controller. The old curl method actually never fully allowed you to open your hand even.
The even more annoying part is that the oculus hand tracking is going to behave differently than the steamvr back end as well in openXR. So you can’t exactly just run the steamvr hands alone and expect that to function as expected.