With the recent SteamVR Input 2.0 the hand posing also changed a lot. The patch notes mentions the finger posing is considering the Humanoid Muscle Settings, but to what extent is unclear.
I have an avatar with each finger's "spread" range set to 0-0 and despite that the hand will still be splayed a significant amount.
On another avatar I had played around with the finger angles to see if it would affect the tracked fingers (Valve Index), with no difference to be seen. I had specifically set the ranges to something realistically unreasonable just to test the limits. So it then surprised me to see its hands being fine with both "Legacy Fingers" and the new Input 2.0 fingers.
So what values are considered from the humanoid setup? As it stands, it looks like nothing is taken into account.