I remember seeing a post here maybe a month or two ago where a member had access to an electron microscope. In his pictures the effects of stropping were definitely visible. This was for me, the first (and only) objective evidence that stropping has an effect on the edge.
If stropping changes the surface on the sub-microscopic level, it would explain why the kind of leather is not too important.
The pictures gave me confidence that stropping was indeed necessary, and also helped me lighten my pressure used to strop. (understanding that if what was going on was at a submicroscopic level, much pressure was not needed- or desired)