I am still musing on this very question ( as it has formed in my mind from having been learning to Work on - or understand - the mechanisms of the Swing-out Cylinder DAs ).
Narrating from my own naïve Mental Model - I t-h-i-n-k the issue resolves on the height to which the Rebound Lever is raised, and held, during Cycling.
Or that if the Rebound Lever is not raised and held high enough, the Revolver may still cycle perfectly, but, the Bolt ( which is moved 'down' on the indexing end, by the Rebound Lever being elevated or raised ) may be just touching or lightly dragging on the Cylinder, thus making the famed 'Drag Line'.
Thus, either the Bolt needs to be a little lower on either or both of it's ends, or, the Rebound Lever needs to be elevated a little more and held there, either by the Rebound Lever itself being a little lower where it rides on the Hand base, or, the Hand Base needs to be a little higher where the Rebound Lever rides on it.
Hopefully wiser Heads than mine will help clarify this question.