Why parented to bone object has offset in local space? Sphere empty is a bone's Сhild (origin is not cleared for it).
Cube empty has CopyLocation constraint set to local space of Sphere empty.




Now Cube has some offset, it is equal to bone's length. But why is it here, if Sphere empty stays right in the world origin
To remove offset from relation:


Clear origin for child object, ALT O (translation only) or clear parent inverse, ALT P (translation+rotation)
In bone's options enable Relative Parenting